[MERGE] Add bzrlib.smart.message module: new abstractions for protocol logic (protocol v3 patch 2/7)

[Andrew Bennetts]

This is the second in a series of patches that implement version 3 of the smart
server protocol.  The full branch with all changes combined is at
<http://people.ubuntu.com/~andrew/bzr/protocol-v3-loom/>

message.py
----------

New abstractions for handling smart server messages.  The previous protocol
logic was implemented basically by a pair of classes:
SmartServerRequestProtocolNNN/SmartClientRequestProtocolNNN.  I've broken these
into smaller pieces:
 - RequestEncoder
 - RequestDecoder
 - ResponseEncoder
 - ResponseDecoder
 - RequestHandler
 - ResponseHandler

This has made testing and reasoning about the code simpler.

The Decoder classes take bytes, and turn them into events that are dispatched to
a MessageHandler.  bzrlib.smart.message implements ConventionalRequestHandler
and ConventionalResponseHandler.  (See the v3 specification in
network-protocol.txt for the meaning of "conventional" in this context.)
ResponseHandler implements a subset of the interface from
SmartClientRequestProtocolOne, the parts useful to client code in
bzrlib.smart.client (and thus to bzrlib.remote and bzrlib.transport.remote).

client.py
---------

Change _build_client_protocol to return separate encoder and handler objects.

-Andrew.

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