Call the extra_watch_callback on a different thread than the dispatcher

Otherwise, a call to e.g. Xs.read will deadlock if you try to do it
from the callback.

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>
Add a test to check that reads work in a watch callback

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>
Merge pull request #8 from jonludlam/watch-callbacks-on-separate-thread

Watch callbacks on separate thread