Only end upgrade socket connections if unhandled

This fixes an issue where multiple engine.io instances (on different
paths) would result in the closing of websocket connections for all of
the instances. This happened because each engine.io instance would
register an `upgrade` handler on the server. This handler would check
for a matching path and otherwise call `socket.end()` Since multiple
upgrade events would be triggered with different paths, the peer
handlers would close each other.

This patch resolves this behavior in the following way:
- When an instance upgrade handler encounters a path which it does not
  recognize it creates a timeout for `destroyUpgradeTimeout`.
- At the end of the timeout, the socket is checked for writable state
  and bytes written. If there has been not activity and the socket is
  writable, then it will be ended.

This allows for peer socket handlers to keep the socket alive by sending
some data over it. This also mimics the core node behavior of closing
sockets on upgrade when no handler is specified. We consider not
handling an upgrade request similar to no handler. However, we cannot
immediately end the socket for the reasons noted above.

fixes #143
Merge pull request #144 from shtylman/fix-multiple-paths

Only end upgrade socket connections if unhandled