Fix minor problems with non-exclusive backup cleanup.

 Fix minor problems with non-exclusive backup cleanup.
 
 The previous coding imagined that it could call before_shmem_exit()
 when a non-exclusive backup began and then remove the previously-added
 handler by calling cancel_before_shmem_exit() when that backup
 ended. However, this only works provided that nothing else in the
 system has registered a before_shmem_exit() hook in the interim,
 because cancel_before_shmem_exit() is documented to remove a callback
 only if it is the latest callback registered. It also only works
 if nothing can ERROR out between the time that sessionBackupState
 is reset and the time that cancel_before_shmem_exit(), which doesn't
 seem to be strictly true.
 
 To fix, leave the handler installed for the lifetime of the session,
 arrange to install it just once, and teach it to quietly do nothing if
 there isn't a non-exclusive backup in process.
 
 This is a bug, but for now I'm not going to back-patch, because the
 consequences are minor. It's possible to cause a spurious warning
 to be generated, but that doesn't really matter. It's also possible
 to trigger an assertion failure, but production builds shouldn't
 have assertions enabled.
 
 Patch by me, reviewed by Kyotaro Horiguchi, Michael Paquier (who
 preferred a different approach, but got outvoted), Fujii Masao,
 and Tom Lane, and with comments by various others.
 
 Discussion: http://postgr.es/m/CA+TgmobMjnyBfNhGTKQEDbqXYE3_rXWpc4CM63fhyerNCes3mA@mail.gmail.com