Use optimized bitmap set function for membership test in postgres_fdw

Use optimized bitmap set function for membership test in postgres_fdw

Deparsing logic in postgres_fdw for locking, FROM clause (alias) and Var
(column qualification) does not need to know the exact number of members
involved, which can be calculated with bms_num_members(), but just if
there is more than one relation involved, which is what bms_membership()
does.  The latter is more performant than the former so this shaves a
couple of cycles.

Author: Daniel Gustafsson
Reviewed-by: Ashutosh Bapat, Nathan Bossart
Discussion: https://postgr.es/m/C73594E0-2B67-4E10-BB35-CDE0E41CC384@yesql.se