fix a race when multicasting to a replay subject /ht @jspahrsummers

The replay subscription could be disposed of before it had a chance to
actually do the replay.
back to using a BOOL to track whether we've connected
don't need to synchronize for the disposable anymore
make the disposable atomic
test autoconnection + replay subject
better test for multicasting to a replay subject
Merge pull request #245 from github/fix-multicast

Fix a race when multicasting to a replay subject