FB 32224. Removes the _replicator database

This commit removes the _replicator database that was used to control
and monitor replication processes. With this update, the TDReplicator
objects are created directly with TDReplicatorManager. The CDReplicator
objects are conceptually no longer wrappers around _replicator documents,
but instead are wrappers around the TDReplicator objects directly.

The CDTReplicator is initialized with TDReplicatorManager object and
a NSDictionary that specifies the properties of the replication when
created by the CDTReplicatorFactory. There are significant changes to
CDTReplicator.m to make it reflect and monitor the state of the underlying
TDReplicator.

References to the _replicator database are removed from TD_DatabaseManager.
Removes support of "touchdb" protocol for a remote URL from TD_DatabaseManager.
Adds TDReplicatorStartedNotification to TDReplicator.
Adds -createReplicatorWithProperties: and -startReplicator: to
TDReplicatorManager.

By removing the _replicator database, however, there is now no support
to restart replications after the application returns from the background
or is restarted/crashed.
They were public to the class, but not in the _replicator document which was accessible externally to the library. I was unclear there. Remember that things that appear "public" in a class don't get exposed via the library-external API much of the time given its HTTP nature in the original TouchDB code.

My guess this was set to zero because it was only there fore _active_tasks, which obviously didn't require it once the replication was complete, and the _replicator document didn't include it until I changed that. So I guess there's no reason for us to zero it.