gc: replace vclockset_psearch with _match in wal_collect_garbage_f

When using vclockset_psearch, the resulting vclock may be incomparable
to the search key. For example, with a vclock set { } (empty vclock),
{0: 1, 1: 10}, {0: 2, 1:11} vclockset_psearch(set, {0:2, 1: 9}) might
return {0: 1, 1: 10}, and not { }.
This is known and avoided in other places, for example
recover_remaining_wals(), where vclockset_match() is used instead.
vclockset_match() starts with the same result as vclockset_psearch() and
then unwinds the result until the first vclock which is less or equal to
the search key is found.

Having vclockset_psearch in wal_collect_garbage_f could lead to issues
even before local space changes became written to 0-th vclock component.
Once replica subscribes, its' gc consumer is set to the vclock, which
the replica sent in subscribe request. This vclock might be incomparable
with xlog vclocks of the master, leading to the same issue of
potentially deleting a needed xlog during gc.

Closes #7584

NO_DOC=bugfix