ofproto-dpif: Simplify commit logic.

ofproto-dpif: Simplify commit logic.

Before executing an output action, ofproto-dpif must commit the
changes it's made to the flow so they are reflected in the
packet.  This code has been unnecessarily complex.  This patch
attempts to simplify the code in the following ways.

- Commit in fewer places.
In an attempt to provide some optimization, the ofproto-dpif code
separated the commit and output composition steps so things like
flood actions could avoid redundant commits.  This is a case of
premature optimization that makes the code significantly more
difficult to reason about.  With this patch, commits happen only
when really necessary.

- Only perform full commits.
In an attempt to provide some optimization, the ofproto-dpif code
would allow callers to only commit the part of the flow that they
had modified by directly calling the relevant subroutine.  This
practice made the code difficult to reason about and is thus
discontinued.

- Perform all output logic in one function.
All of the logic surrounding the datapath output action has been
placed in the compose_output_action__() function.  Most callers
will use the compose_output_action() function which simply passes
reasonable defaults through to compose_output_action__().