Switch various builtin functions to use pg_lsn instead of text.

The functions in slotfuncs.c don't exist in any released version,
but the changes to xlogfuncs.c represent backward-incompatibilities.
Per discussion, we're hoping that the queries using these functions
are few enough and simple enough that this won't cause too much
breakage for users.

Michael Paquier, reviewed by Andres Freund and further modified
by me.
Document pg_replslot in storage.sgml.

Per an observation from Amit Kapila.