cleanup old WAL segments when the WAL PVC utilization reaches more than 90%

[boddumanohar]

fixes: simplyblock/vela#257

cleanup old WAL segments when the WAL PVC utilization reaches more than 90%.

A periodic celery beat task that checks the utilisation of WAL and trigger the wal_cleanup task.

the task gets the Safe WAL checkpoint

SELECT pg_walfile_name(redo_lsn) FROM pg_control_checkpoint();

and cleanup the WAL files older than this by running

pg_archivecleanup /mnt/postgres_wal/wal $SAFE_WAL