refactor RGWClientIO

refactor RGWClientIO

In order to support direct/library clients, we want to
more fully decouple RGWOp processing from client HTTP
processing, since in the direct case, there is no HTTP
client.

This change tackles essentially just RGWClientIO, by moving
stream operations to a new subclass RGWStreamIO, and updating
the stream backends (e.g., SWIFT, S3) and REST handlers
accordingly.

In addition, the RGWLib backend is incrementally updated, with
RGWLibIO still deriving from the base RGWClientIO.  The RGWLib
path is incomplete as of this change, but is moving in the
direction of its own process_request call path (incomplete,
partly due to header conflicts that will be resolved in
subsequent commits).

Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>