move EngineEvents to ConnectionEvents, allow more scopes

[Mike Bayer]

want to allow event dispatch per connection, per engine, on Connection, and Engine. the dispatch really should be on Connection. _listen() can set up a simple flag on the connection or cls telling it to care about dispatch or not. dispatch for a specific Engine can generate a subclass of Connection in _listen so that it gets its own dispatch.

[Mike Bayer]

the attached patch is me beginning to screw around with this.

[Mike Bayer]

OK things have been simplified, if not slightly more verbose, in 7dab4ae6a9ca057bb99be2f01efc26610be12f63. For the moment I don't think I want to add per-connection events, though this architecture will allow for it more closely - basically Connection would have to iterate through two different dispatchers, or otherwise we would need to combine dispatchers at connect time somehow. For now the overhead stays low.

[Mike Bayer]

Removing milestone: 0.7.0 (automated comment)