tornado: Split AsyncSentryClient in sync/async

Motivation is as follows:

Tornado is partially supported on Google App Engine (https://github.com/facebook/tornado/tree/master/demos/appengine). GAE's sandbox won't allow
tornado's async support, so importing tornado.ioloop results in this
error:

  File "/home/vagrant/workspace/myproject/lib/python2.7/site-packages/tornado/platform/posix.py", line 21, in <module>
    import fcntl
  File "/opt/google_appengine/google/appengine/tools/devappserver2/python/sandbox.py", line 822, in load_module
    raise ImportError('No module named %s' % fullname)
ImportError: No module named fcntl

Thus, without this patch, the contrib.tornado module can't be imported
in such environments. For other environments nothing changes and this
should be backwards compatible. Applications running tornado on GAE
can import this module and subclass contrib.tornado.SentryClient and
implement the http client part using the urlfetch service (
https://developers.google.com/appengine/docs/python/urlfetch/).