Introduce a new pgcopydb internal message: ENDPOS. (#321)

* Introduce a new pgcopydb internal message: ENDPOS.

When reaching endpos in the middle of a transaction we should stop
processing the stream, transform what we have, and apply up to the last full
transaction, ignoring the partial one at the end.

Because of the streaming approach taken by pgcopydb, the next best thing we
can do is to ROLLBACK the last partial transaction while still being able to
recognise that --endpos has been reached.

Previously to this patch, pgcopydb would use the endpos LSN to forge a
keepalive message, meaning that we would then update our replay_lsn and
replication origin tracking to a position in the middle of a transaction we
did not replay.

In case when the user then restart pgcopydb with a new endpos then we would
skip one transaction which was written off as already replayed. This patch
fixes this situation.

* Fix resuming CDC after chaging endpos again.

It should be possible to reset endpos to a later point in time and resume
replaying changes up to the new point in time, even when the previously set
endpos did fell in the middle of a transaction.

* Fix comparing with startpos from replication slot.

When comparing startpos with a previously written JSON file we need to avoid
writing the same LSN twice, but that computation/guard doesn't apply when
starting CDC the first time from the replication slot: then we want to write
starting at the startpos, not skip it.