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.
It turns out that TDReplicators is not resumable. Check out https://github.com/cloudant/CDTDatastore/blob/32224-remove-_replicator-database/Classes/common/touchdb/TDReplicator.m#L206. 

The _db pointer is set to nil at the end of -stop:  https://github.com/cloudant/CDTDatastore/blob/32224-remove-_replicator-database/Classes/common/touchdb/TDReplicator.m#L310.

I believe that I have made CDTReplicator resumable, but I need to write a test for that to be sure.