indexes: Add compact block filter headers cache
Summary:
Cache block filter headers at heights of multiples of 1000 in memory. Block filter headers at height 1000x are checkpointed, and will be the most frequently requested. Cache them in memory to avoid costly disk reads.
Backport of core PR18960.
Depends on D8463.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D8464