In 2ea0daab19, handlers were moved to
being tracked via the handlers_by_id dict, but nothing cleared this
dict, resulting in every handler object being leaked. Since a Tornado
process uses a different handler object for every request, this
resulted in a significant memory leak. We fix this by clearing the
handlers_by_id dict in the two code paths that would result in a
Tornado handler being de-allocated: the exception codepath and the
handler disconnect codepath.
Fixes#463.