Refactor: RPC: Separate GetBlockChecked() from getblock()

Summary:

This does not change functionality

Partial backport of core PR10757 (commit cda8e36)
https://github.com/bitcoin/bitcoin/pull/10757/commits/cda8e36f019dd181e5c3774961b4f1335e5602cb

Test Plan:

make check
./test/functional/test_runner.py rpc_blockchain

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3755