* `CConnman::CalculateKeyedNetGroup()` needs `CNetAddr`, not `CAddress`,
  thus change its argument.

* Both callers of `CConnman::CreateNodeFromAcceptedSocket()` create a
  dummy `CAddress` from `CService`, so use `CService` instead.

Introduce a new low-level socket managing class `SockMan`
and move the `CConnman::BindListenPort()` method to it.

Also, separate the listening socket from the permissions -
they were coupled in `struct ListenSocket`, but the socket
is protocol agnostic, whereas the permissions are specific
to the application of the Bitcoin P2P protocol.

It was copied verbatim from `CConnman::BindListenPort()` in the previous
commit. Modernize its variables and style and log the error messages
from the caller.

Move the `CConnman::AcceptConnection()` method to `SockMan` and split
parts of it:
* the flip-to-CJDNS part: to just after the `AcceptConnection()` call
* the permissions part: at the start of `CreateNodeFromAcceptedSocket()`

CConnman-specific or in other words, Bitcoin P2P specific. Now
the `ThreadI2PAcceptIncoming()` method is protocol agnostic and
can be moved to `SockMan`.

Change `CConnman::m_nodes` from `std::vector<CNode*>` to
`std::unordered_map<NodeId, CNode*>` because interaction
between `CConnman` and `SockMan` is going to be based on
`NodeId` and finding a node by its id would better be fast.

As a nice side effect the existent search-by-id operations in
`CConnman::AttemptToEvictConnection()`,
`CConnman::DisconnectNode()` and
`CConnman::ForNode()` now become `O(1)` (were `O(number of nodes)`),
as well as the erase in `CConnman::DisconnectNodes()`.

Move the parts of `CConnman::GenerateWaitSockets()` that are specific to
the Bitcoin-P2P protocol to dedicated methods:
`ShouldTryToSend()` and `ShouldTryToRecv()`.

This brings us one step closer to moving `GenerateWaitSockets()` to the
protocol agnostic `SockMan` (which would call `ShouldTry...()` from
`CConnman`).

…cketHandler()

Move some parts of `CConnman::SocketHandlerConnected()` and
`CConnman::ThreadSocketHandler()` that are specific to the Bitcoin-P2P
protocol to dedicated methods:
`EventIOLoopCompletedForNode()` and `EventIOLoopCompletedForAllPeers()`.

This brings us one step closer to moving `SocketHandlerConnected()` and
`ThreadSocketHandler()` to the protocol agnostic `SockMan` (which would
call `EventIOLoopCompleted...()` from `CConnman`).

Introduce 4 new methods for the interaction between `CConnman` and
`SockMan`:

* `EventReadyToSend()`:
  called when there is readiness to send and do the actual sending of data.

* `EventGotData()`, `EventGotEOF()`, `EventGotPermanentReadError()`:
  called when the corresponing recv events occur.

These methods contain logic that is specific to the Bitcoin-P2P protocol
and move it away from `CConnman::SocketHandlerConnected()` which will
become a protocol agnostic method of `SockMan`.

Also, move the counting of sent bytes to `CConnman::SocketSendData()` -
both callers of that method called `RecordBytesSent()` just after the
call, so move it from the callers to inside
`CConnman::SocketSendData()`.

Move the protocol agnostic parts of `CConnman::ConnectNode()` into
`SockMan::ConnectAndMakeNodeId()` and leave the Bitcoin-P2P specific
stuff in `CConnman::ConnectNode()`.

Move the protocol agnostic `CConnman::m_unused_i2p_sessions`, its mutex
and `MAX_UNUSED_I2P_SESSIONS_SIZE` to `SockMan`.

Move `GetBindAddress()` from `net.cpp` to `sockman.cpp`.

Move `MaybeFlipIPv6toCJDNS()`, which is Bitcoin P2P specific from the
callers of `CConnman::EventNewConnectionAccepted()` to inside that
method.

Move the IsSelectable check, the `TCP_NODELAY` option set and the
generation of new node id out of `CConnman::EventNewConnectionAccepted()`
because those are protocol agnostic. Move those to a new method
`SockMan::NewSockAccepted()` which is called instead of
`CConnman::EventNewConnectionAccepted()`.

Move `CNode::m_sock` and `CNode::m_i2p_sam_session` to `SockMan::m_connected`.
Also move all the code that handles sockets to `SockMan`.

`CNode::CloseSocketDisconnect()` becomes
`CConnman::MarkAsDisconnectAndCloseConnection()`.

`CConnman::SocketSendData()` is renamed to
`CConnman::SendMessagesAsBytes()` and its sockets-touching bits are moved to
`SockMan::SendBytes()`.

`CConnman::GenerateWaitSockets()` goes to
`SockMan::GenerateWaitSockets()`.

`CConnman::ThreadSocketHandler()` and
`CConnman::SocketHandler()` are combined into
`SockMan::ThreadSocketHandler()`.

`CConnman::SocketHandlerConnected()` goes to
`SockMan::SocketHandlerConnected()`.

`CConnman::SocketHandlerListening()` goes to
`SockMan::SocketHandlerListening()`.

`SockMan` members

`AcceptConnection()`
`NewSockAccepted()`
`GetNewNodeId()`
`m_i2p_sam_session`
`m_listen private`

are now used only by `SockMan`, thus make them private.

Co-Authored-By: Christopher Coverdale <[email protected]>

Move the implementation (method definitions) from `test/util/net.h` to
`test/util/net.cpp` to make the header easier to follow.

…lass

This allows reusing them in other mocked implementations.

…o it

And also allows gradually providing the data to be returned by `Recv()`
and sending and receiving net messages (`CNetMessage`).

…tion

Co-Authored-By: Vasil Dimov <[email protected]>