Rename all 1MB limits in qa tests to LEGACY_MAX_BLOCK_BASE_SIZE
Summary:
This is prep work so that we know where we need to review and adapt in qa tests,
as we roll out >1MB capability.
Test Plan: qa/pull-tester/rpc-tests -extended
Reviewers: #bitcoin_abc, deadalnix, sickpig, kyuupichan
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D153