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