index: Access functions for global block filter indexes.

Summary:
This is a partial backport of Core PR14121 : https://github.com/bitcoin/bitcoin/pull/14121/commits/accc8b8b1842ed6d522b71a056777bcac8f39e81

Depends on D6341 and D6344

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6345