Reverse database url MTA option

Memcache/Redis MTA options:  (memcache:/redis: URL)
The value of the  MTA option should be  either a memcache: or (as of MS 8.1) a redis:URL for storing  reverse database data. Note that  will only be consulted if bit 1 (value 2) of the    MTA option is clear (not set); setting    to a value subsuming 2 causes any setting of   to be ignored.

See also:
 * Database MTA options
 * Reverse database
 * MTA URL types
 * use_text_databases MTA Option
 * Memcache MTA options
 * Redis MTA options