Merge lp://staging/~free.ekanayaka/txamqp/closing-logic-cleanup into lp://staging/txamqp

This branch implements the following changes:

- TwistedDelegate.connection_close now sends a 'close-ok' method frame before calling AMQClient.doClose. There's no response for this type of method, but it will be confident that it will be written to the transport before closing it, since AMQClient.doClose eventually calls transport.loseConnection(), which does flush buffers before shutting down the socket.

- AMQClient now has a new "disconnected" instance attribute, which is a TwistedEvent that consumer code can wait on in order to know when the transport has actually shut down. This is useful for integrating txamqp with things like twisted.application.internet.ClientService (that would start to connect again as soon as the connection is lost).

- AMQClient.closed is now properly initialized in the __init__ (it was set dynamically before), so it's always safe to inspect it.

- The AMQClient.sendHB LoopingCall now gets the same clock as AMQClient, instead of the global reactor. This makes unit-testing easier.

- AMQClient.queues (of type TimeoutDeferredQueue) also get same clock as AMQClient.

- The "reason" parameter of AMQClient.close is now optional and defaults to ConnectionDone.

- AMQClient.close has a new "within" parameter, which tells the client to possibly try to perform a clean connection shutdown with the 'close'/'close-ok' frames handshake. If the handshake doesn't succeed in "within" seconds, the transport connection gets terminated without handshake. The default of the new parameter is 0, which means "do close immediately without handshake", so there's should be no behavior change at all for existing code, expect that close() now returns a Deferred, but it seems minor, since the non-handshake code is still all synchronous.

- The old AMQClient.close logic has been moved to AMQClient.doClose, which is basically the same pattern as AMQChannel.close/doClose.

- AMQClient.checkHeartbeat now calls abortConnection rather than loseConnection: since the peer is unresponsive it most probably won't perform the TCP closing handshake anyways.

- Unit tests and integration tests where added for all the behavior above.

Phew, that was quite a bit of narrative, really hope the diff isn't controversial :)

After this branch I have a final small one that allows application code to distinguish between connection close errors and channel close errors (with two new subclasses of Close). Other than that I have no immediate pending work on txamqp, but I'd like to resume:

https://code.launchpad.net/~ei-grad/txamqp/publish-confirms

and have support for confirms.

Thanks!