Sync signatures of gbt_var_consistent() and gbt_num_consistent().

Sync signatures of gbt_var_consistent() and gbt_num_consistent().

For some odd reason we pass the strategy number to gbt_num_consistent
as "const StrategyNumber *strategy".  There's no reason for that:
it almost certainly costs more at both callers and callee to pass a
pointer than to pass a small integer value.  And it's inconsistent
with gbt_var_consistent(), so fix it.

gbt_var_consistent() had its own infelicity, which was not marking
the input "key" value const.  Fix that too while we're here.

This is primarily cosmetic, so I see no need to backpatch.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/AH*AvQCYKhQGVvPWi1GiU4oY.8.1781609375063.Hmail.3020001251@tju.edu.cn