pgsql: Add pgstat_drop_matching_entries() to pgstats

Add pgstat_drop_matching_entries() to pgstats

This allows users of the cumulative statistics to drop entries in the
shared hash stats table, deleting as well local references. Callers of
this function can optionally define a callback able to filter which
entries to drop, similarly to pgstat_reset_matching_entries() with its
callback do_reset().

pgstat_drop_all_entries() is refactored so as it uses this new function.

Author: Lukas Fitti
Discussion: https://postgr.es/m/CAP53PkwuFbo3NkwZgxwNRMjMfqPEqidD-SggaoQ4ijotBVLJAA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ce5c620fb6252dca00d3856d5f09d56c7f1215d0

Modified Files
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src/backend/utils/activity/pgstat_shmem.c | 31 ++++++++++++++++++++++++++++++-
src/include/utils/pgstat_internal.h | 2 ++
2 files changed, 32 insertions(+), 1 deletion(-)