Temporary WAL segments are created in pg_wal and named as xlogtemp.pid
before being renamed to the real deal when creating a new segment.  If
an instance crashes after the temporary segment is created and before
the rename is done, then the server would finish with unremovable data.

After an instance crash, scan pg_wal and remove any such segments.  With
repetitive unlucky crashes this would contribute to disk bloat and
presents risks of ENOSPC especially with max_wal_size close to the
maximum allowed.

Author: Michael Paquier
Reviewed-by: Yugo Nagata, Heikki Linnakangas
Discussion: https://postgr.es/m/20180514054955.GF1528@paquier.xyz