Update pgcopydb sentinel in the main follow process. (#521)

* Update pgcopydb sentinel in the main follow process.

To avoid infinite looping when endpos has been reached it's important to
update our endpos value to the sentinel's one even in the main follow
process.

In passing, review some error messages.

Also refrain from stopping early from reading data from a PIPE when a
terminating signal is received, we should finish reading as per the comments
in the code.

* Process ENDPOS in transform and replay processes.

When the ENDPOS internal message is received from the PIPEs in replay mode,
check the current sentinel.endpos value on the source database to make sure
the ENDPOS message matches with the current setting, and that being the case
stop processing.