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.
I thought about using NSLock (-tryLock) instead, but reasoned that @synchronized(self) was better.The only advantage that -tryLock would provide is it could return NO immediately instead of waiting for the first thread that got there. But since the code block is tiny, this would save about  1/100000000000 th of a second.