Refactors ClientInterface and Guzzle adapter.

This patch achieves three interrelated changes

1. an update to the ClientInterface, making it more efficient and
consistent with other HTTP libraries. PSR-FIG messages have also been
added as separate classes, rather than lumped as one. Guzzle
functionality has also been moved to its own namespace;

2. the refactoring of `GuzzleClient` to `GuzzleAdapter`, including
relevant changes needed for the interface change (1). We now have
ADAPTERS that bridge our interfaces with Guzzle's - making that
difference much clearer, extensible, less tightly coupled and less
brittle;

3. moving "bad request" error handling into its own dedicated space,
based on how the new Adapter functionality (2). Previously the client
tried to do all the exception logic - but this is not strictly its
responsibility. This logic has been shipped out to a base
RequestException which instantiates one of its children exception based
on the HTTP status code.

Although I have attempted to keep the scope of this patch to a minimum,
as granular as possible, because the interface is a core internal API,
various other files need to be modified to reflect the change. In terms
of the other two changes, these are inextricably linked to the interface
change, so cannot be teased out into their own patches.

Change-Id: Ibc1b50cec125c11d32ee6e4f0dbb395fcaea864e