Move VerifySignature to BaseSignatureChecker and use it for OP_CHECKDATASIG

Summary: This ensure that we also benefit from signature caching facilities provided by CachingTransactionSignatureChecker.

Test Plan:

make check
./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1666