Fix minor bug in logical-replication walsender shutdown

Logical walsender should exit when it catches up with sending WAL during
shutdown; but there was a rare corner case when it failed to because of
a race condition that puts it back to wait for more WAL instead -- but
since there wasn't any, it'd not shut down immediately.  It would only
continue the shutdown when wal_sender_timeout terminates the sleep,
which causes annoying waits during shutdown procedure.  Restructure the
code so that we no longer forget to set WalSndCaughtUp in that case.

This was an oversight in commit c6c33343.

Backpatch all the way down to 9.4.

Author: Craig Ringer, Álvaro Herrera
Discussion: https://postgr.es/m/CAMsr+YEuz4XwZX_QmnX_-2530XhyAmnK=zCmicEnq1vLr0aZ-g@mail.gmail.com