re-work the multiprocessin encryption/decryption pool
right now, it has too many smells that point to a bad design (tm). for instance, conditionals to check whether the encryption/decrytion is delegated to the multiprocessing pool is spread across too many methods, spawning different modules. also, it's not trivial to test it.
beware that it shoudln't block the reactor!
(from redmine: created on 2015-12-01)