Feature/enc sync transitional db

Optionally move the encryption and decryption of the syncing docs to an intermediate local db, and process the encryption/decryption using python multicore capabilities provided by multiprocessing (and thus bypassing the dreaded GIL).

soledad.client.crypto refactor is taking functions to the module level to avoid pickling the SoledadCrypto object while dispatching encryption to the pool of workers (multiprocessing.Pool)