Cleanup commit timestamp module activaction, again

Further tweak commit_ts.c so that on a standby the state is completely
consistent with what that in the master, rather than behaving
differently in the cases that the settings differ.  Now in standby and
master the module should always be active or inactive in lockstep.

Author: Petr Jelínek, with some further tweaks by Álvaro Herrera.

Backpatch to 9.5, where commit timestamps were introduced.

Discussion: http://www.postgresql.org/message-id/5622BF9D.2010409@2ndquadrant.com