Sieve filter caching MTA options

At the intersection between Sieve filter processing and  MTA caching of fetched data,  are a couple of MTA options.

See also:
 * Sieve filter MTA options
 * LDAP lookup cache MTA options
 * filter_cache_size MTA Option
 * filter_cache_timeout MTA Option
 * Sieve filters
 * MTA options