Fix SetClientEncoding() to maintain a cache of previously selected encoding

conversion functions.  This allows transaction rollback to revert to a
previous client_encoding setting without doing fresh catalog lookups.
I believe that this explains and fixes the recent report of "failed to commit
client_encoding" failures.

This bug is present in 8.3.x, but it doesn't seem prudent to back-patch
the fix, at least not till it's had some time for field testing in HEAD.

In passing, remove SetDefaultClientEncoding(), which was used nowhere.