OFStateManager: remove error return from message handlers

These never did anything useful - they just ended up (as an integer) in a
generic log message. Later commits will improve error logging; I tried to keep
other changes to a minimum.
OFStateManager: remove useless wrapper IND_CORE_MSG_SEND
OFConnectionManager: remove return value from indigo_cxn_send_controller_message

There's no way to handle this failure besides logging it (or disconnecting the
controller), which OFConnectionManager will already do.
OFConnectionManager: replace indigo_cxn_send_error_msg with indigo_cxn_send_error_reply

This function takes the request object as a parameter, removing the need to
pass octets, xid, version, etc.

It also does not return an error to the caller, because there's nothing the
caller can do about it.
OFStateManager: remove unused message handlers

These messages are always handled by OFConnectionManager.
OFStateManager: remove redundant log messages

Both indigo_core_receive_controller_message and OFConnectionManager already log
the same information.
OFStateManager: remove clutter from message handlers
OFStateManager: replace unused message handlers with calls to ind_core_unhandled_message
OFStateManager: demote unhandled message error to a warning

This is not necessarily an error. A controller might just be trying to use an
extension we don't support.
indigo: add indigo_strerror

This involves redefining the error codes as an x-macro and instantiating it for
both the enum and the new indigo_strerror function.
OFStateManager: improve logging

Mostly added uses of aim_strerror. Also removed some extra newlines in log
messages.
OFConnectionManager: improve logging

Added uses of strerror and aim_strerror. Replaced integer cxn ids with string
addresses. Added XIDs to message send/recv logs.
OFStateManager: remove redundant logs
OFConnectionManager: use cxn_ip_string in message traces
OFConnectionManager: create API for indigo_cxn_send_async_message

This function was previously invoked by passing UNSPECIFIED as the cxn id.
OFStateManager: remove unused cxn_id param from ind_core_flow_entry_delete
OFStateManager: remove ind_core_xid_alloc

Flow-removed messages don't need XIDs.
OFStateManager: get flow-removed msg version from OFConnectionManager
OFStateManager: remove unused ft_delete_id
OFStateManager: ft_delete cannot fail