This change makes the commit index advancement in handleHeartbeat safe.
Previously, a follower would attempt to update the commit index to
whichever was sent in the MsgHeartbeat message. Out-of-bound indices
would crash the node.

It is always safe to advance a commit index if the follower's log is "in
sync" with the leader, i.e. when its log is guaranteed to be a prefix of
the leader's log. This is always true if the term of last entry in the
log matches the leader team, otherwise this guarantee is established
when the first MsgApp append message from the leader succeeds.

At the moment, the leader will never send a commit index that exceeds
the follower's log size. However, this may change in future. This change
is a defence-in-depth.

The newly added raftLog.leaderTerm field will be used for other safety
checks in the future, for example to establish that overriding a suffix
of entries in raftLog is safe.

Signed-off-by: Pavel Kalinnikov <[email protected]>

This change improves safety of the append operations on raftLog. This
helped fixing some tests which made incorrect assumptions about the log.

Signed-off-by: Pavel Kalinnikov <[email protected]>