Fix performance regression in dblink connection speed.

Previous commit e5de601267d98c5d60df6de8d436685c7105d149 modified dblink
to ensure client encoding matched the server. However the added
PQsetClientEncoding() call added significant overhead. Restore original
performance in the common case where client encoding already matches
server encoding by doing nothing in that case. Applies to all active
branches.

Issue reported and work sponsored by Zonar Systems.