pg_receivewal: Add --endpos option

pg_receivewal: Add --endpos option

This is primarily useful for making tests of this utility more
deterministic, to avoid the complexity of starting pg_receivewal as a
deamon in TAP tests.

While this is less useful than the equivalent pg_recvlogical option,
users can as well use it for example to enforce WAL streaming up to a
end-of-backup position, to save only a minimal amount of WAL.

Use this new option to stream WAL data in a deterministic way within a
new set of TAP tests.

Author: Michael Paquier <michael.paquier@gmail.com>