Add btree and hash opclasses for pg_lsn.

This is needed to allow ORDER BY, DISTINCT, etc to work as expected for
pg_lsn values.

We had previously decided to put this off for 9.5, but in view of commit
eeca4cd35e284c72b2ea1b4494e64e7738896e81 there's no reason to avoid a
catversion bump for 9.4beta2, and this does make a pretty significant
usability difference for pg_lsn.

Michael Paquier, with fixes from Andres Freund and Tom Lane