TailDirSource causes master to fail if directory doesn't exist

TailDirSource creates a DirWatcher in the constructor. If the directory doesn't exist on this master an exception will be thrown on the master - rather than what was really intended - that the check be done on the destination node.

jon says:

> The right thing to do is either move the precondition from the constructor to the
> start method, or move the instantiation of the DirWatcher to the open call.