datapaths: Use OpenFlow constants directly

This had been being done with the reverse maps, which is
not necessary.
datapaths: Clean up asserts
datapaths.switch: 80 column formatting
datapaths.switch: Honor max buffer count

We no longer allocate a new buffer if we already have the maximum number
allocated.  Instead, we return None.
datapaths.switch: Rename _receive_echo to _receive_echo_request

All names are now consistent with their OFPT_ counterparts.
datapaths.switch: Improve convention adherence

This updates function defs and docstrings to conform more
closely with POX coding conventions.
datapaths.switch: Remove dated comment
datapaths: All receive take connection

Previously, some receive functions took a connection argument and some
didn't, with some introspection used to figure it out at runtime.  Now
they just all take a connection argument all the time whether they use
it or not.
datapaths.nx_switch: Cleanup

This mostly just improves POX coding convention adherence.
datapaths: Rename on_message_received

POX calls this sort of method an "rx" method.