Make the 'tailerThreads' member final

'tailerThreads' is never changed after construction, although the content may change when the river
is started or stopped. This is however not covered by the 'volatile' keyword, and there is really no
need to allow concurrent modifications of the content (at this point).
Merge pull request #425 from Collaborne/tailer-threads-final

Make the 'tailerThreads' member final