Move transaction's context dependent checks in its own function.

Summary: Right now the only context dependent check is IsFinalTx , but we want to add more in order to ensure replay protection. As a first step, we need a place where these checks can be added. ContextualCheckTransaction is that place.

Test Plan:

make check

Reviewers: freetrader, sickpig, kyuupichan, #bitcoin_abc

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D138