Optimize RelationFindReplTupleSeq() for CLOBBER_CACHE_ALWAYS.
Specifically, remember lookup_type_cache() results instead of retrieving
them once per comparison. Under CLOBBER_CACHE_ALWAYS, this reduced
src/test/subscription/t/001_rep_changes.pl elapsed time by an order of
magnitude, which reduced check-world elapsed time by 9%.
Discussion: https://postgr.es/m/
20200406085420.GC162712@rfd.leadboat.com