Creates individual threads for each TDReplicator.

This commit moves all of the threading code from TDReplicatorManager
to TDReplicator so that each TDReplicator executes on its own thread.

No tests are yet written to monitor simultaneous replications.
Fixes security issue where passwords were being sent to the log.
Adds test for multithreaded replication.

Two pull replications are executed simultaneously, pulling
from the same remote database to two different local databases.
A delegate captures updates from the replicators and determines
if the replicators were operating simultaneously.
Adds access to and tests of the TDReplicator NSThread object.

Can access the TDReplicator's replicator thread's execution state.
Also exposed in CDTReplicator.

Adds to tests that examine the replicator thread's execution state
which verify the thread exits and cancels appropriately.
Merge pull request #82 from cloudant/38257-Move-thread-from-TDReplicatorManager-to-TDReplicator

38257 move thread from td replicator manager to td replicator