Forward 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  forward database data. Note that  will only be consulted if bit 2 (value 4) of the    MTA option is clear (not set); setting    to a value subsuming 4 causes any setting of   to be ignored.

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