Race condition in new NIO based TailSource

Using the test from FLUME-218, I've encountered a race after about several 10k's of events from a single tail source. The resulting behavior duplicates data (which is not ideal but acceptable).

Here's how to duplicated the result:

$ flume node -1 -s -n foo -c 'foo:tail("/tmp/foofile","true")|text("/tmp/foofile.out");'

$ ./short-generate.py >> /tmp/foofile

To view results:
$ wc /tmp/foofile.out
$ sort -n /tmp/foofile.out