General database url MTA option

Memcache/Redis MTA options:  (memcache:/redis: URL)
The value of the  MTA option should be either a fully specified memcache: or (as of MS 8.1) a fully specified redis:URL for storing  general database data. For example, the URL "memcache://memcache.exmaple.com/" specifies that memcache protocol should be used to connect to memcache.exmaple.com on the default memcache port. Note that in the case of Redis the host to connect to is always given by the redis.hostlist option, so the URL is always "redis:///".

Note that the  option will only be consulted if bit 0 (value 1) of the    MTA option is clear (not set); setting    to a value subsuming 1 causes any setting of   to be ignored.

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