Simplifies the TDReplicator threading code.

Left over from NSThread code migration from the
TDReplicatorManager to TDReplicator were some code that
facilitated queueing individual TDReplicator tasks
onto the TDReplicatorManager's thread. Now each
TDReplicator has it's own thread, yet this queueing code
was still left in. This commit removes these parts so
that once TDReplicator -start is called, replication begins
almost immediately.