Fix obsolete statement in smgr/README.

Since commit 2d00190495b22e0d0ba351b2cda9c95fb2e3d083, fork numbers are
defined in relpath.h not relfilenode.h.

Fabrízio de Royes Mello
Reword the sentence for pg_logical_slot_peek_changes function.

Previously the duplicated paragraphs were used next to each other
in the document to demonstrate that the changes in the stream
were not consumed by pg_logical_slot_peek_changes function.
But some users misunderstood that the duplication of the same
paragraph was just typo. So this commit rewords the sentence in
the latter paragraph for less confusing.

Christoph Moench-Tegeder
Treat 2PC commit/abort the same as regular xacts in recovery.

There were several oversights in recovery code where COMMIT/ABORT PREPARED
records were ignored:

* pg_last_xact_replay_timestamp() (wasn't updated for 2PC commits)
* recovery_min_apply_delay (2PC commits were applied immediately)
* recovery_target_xid (recovery would not stop if the XID used 2PC)

The first of those was reported by Sergiy Zuban in bug #11032, analyzed by
Tom Lane and Andres Freund. The bug was always there, but was masked before
commit d19bd29f07aef9e508ff047d128a4046cc8bc1e2, because COMMIT PREPARED
always created an extra regular transaction that was WAL-logged.

Backpatch to all supported versions (older versions didn't have all the
features and therefore didn't have all of the above bugs).
Oops, fix recoveryStopsBefore functions for regular commits.

Pointed out by Tom Lane. Backpatch to 9.4, the code was structured
differently in earlier branches and didn't have this mistake.