Alias database url MTA option

Memcache/Redis MTA options:  (memcache:/redis: URL)
The value of the  MTA option should be a either a memcache: URL or (as of MS 8.1) a redis: URL for storing  alias database data.

See also:
 * Database MTA options
 * Aliases
 * Alias database
 * MTA URL types
 * Memcache MTA options
 * Redis MTA options