add lock_height to Transaction and TxKernel

[antiochp]

Initial attempt at introducing "time locked" transactions.

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TODO -

• rename transaction_fee_and_lock_height_to_bytes -> kernel_sig_msg
• add tests to cover adding invalid tx to pool
• add tests to cover building invalid block

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• add lock_height to Transaction and TxKernel
• msg being signed in a kernel now includes both fee and lock_height
• verify on kernel checks both fee and lock_height against the signature
• wallet sender sets current chain height as the lock_height (as proof of concept)
• pool checks the tx is valid (in terms of lock_height) before adding to pool
• block is invalid if built with a tx with lock_height in future is included

Note: this is similar to the lock_height on a coinbase output but not the same.
This is a "timelock" on the tx/kernel itself, not an output.

See #25 for more info/context/discussion.

[antiochp]

working on fixing up those pool test failures

[ignopeverell]

Good in principle. I'm a little bummed we have to have another clear value in addition to fees but I don't have a better option for now.

[antiochp]

@ignopeverell any further feedback/thoughts on this?
I added some test coverage for both adding timelocked txs to the pool and building a block from timelocked txs.

[ignopeverell]

Looks good to me!