create leap_mx postfix lookup against couchdb for client smtp fingerprint

We need shim code that would run on the postfix server and would do a couchdb query for a particular fingerprint, and answering answer yes or no depending on if the fingerprint exists or not. It needs to be written in an unblocking manner so it can be called a lot

(from redmine: created on 2013-10-24, closed on 2015-12-09, relates #7362, relates #6729, relates #4709, relates #7690)