Changes the CDTReplicator -start signature.

-(void)start becomes -(BOOL)start:NSError**

Errors created by TDReplicator objects are propaged from
TDReplicatorManager. They are reported to the Log (via Warn),
but new CDT-level errors are reported to developers using the
CDT API.
Pretty sure there's a threading issue here. Two threads could call start at the same time, pass through the if statement and end up buggering things up, creating two TD replicators and so on. I don't think you've broken this much more than it was already, but there's an opportunity to fix it when adding the `alreadyStarted` guard I think.

I don't think the rest of the class needs worry about threads, but you probably want a [synchronized](https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/Multithreading/ThreadSafety/ThreadSafety.html#//apple_ref/doc/uid/10000057i-CH8-SW3) block around lines 102 to 117 inclusive.