Serve cfcheckpt requests
Summary:
Serve cfcheckpt messages if basic block filter index is enabled and -peercfilters is set. NODE_COMPACT_FILTERS is not signaled to peers, but functionality can be used for testing and serving pre-configured clients.
Backport of core PR18877.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D8463