(spam-cache-lookups, spam-caches, spam-clear-cache):

(spam-cache-lookups, spam-caches, spam-clear-cache):
first attempt at some caching support (done for BBDB only now)
(spam-find-spam): set spam-cache-lookups if there are more than 2
addresses to be checked
(spam-clear-cache-BBDB): new function, to be invoked by
bbdb-change-hook, and triggering spam-clear-cache of 'spam-use-BBDB
(spam-check-BBDB): check and use the caches, if
spam-cache-lookups is on
remove superfluous (provide)