datapaths: Update default features

We weren't setting some action bits for some actions we supported.
datapaths: Only send one hello

We should actually send a hello on connect instead of replying to hellos.
datapaths: Rework logging for rx methods

There's now a single log statement in the main RX.
datapaths: Improve errors for port mods

We now set the XID and data fields in errors sent in response to port_mods.
datapaths: Log port up/down mods
datapaths: Remove send_echo()

This method was trivial; there's no point simply wrapping the creating and
sending of a message.
datapaths: Improve vendor errors

Set XID and data for errors in response to OFPT_VENDOR.
datapaths: Add TODO list
datapaths: Remove XID generator

Send all packet_ins with XID=0.  I think this is consistent with OVS.

Also update tests for this change.