Add CMerkleTx::IsImmatureCoinBase method

Summary:
All but one call to GetBlocksToMaturity is testing it relative to 0
for the purposes of determining whether the coinbase tx is immature.
In such case, the value greater than 0 implies that the tx is coinbase,
so there is no need to separately test that status.

This names the concept for easy singular use.

Backport of Core PR13631

Test Plan:

make VERBOSE=1 check && ./test/functional/test_runner.py

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1973