OFConnectionManager: always return a version from indigo_cxn_get_async_version

Even if no controller is connected the packet-in still needs to go to the
agents.

It's clear now that we should replace indigo_core_packet_in, and similar
functions, with interfaces that are version-independent.
OFConnectionManager: add IND_CXN_DEFAULT_ASYNC_MESSAGE_VERSION macro
Merge into master from pull request #100:
OFConnectionManager: always return a version from indigo_cxn_get_async_version (https://github.com/floodlight/indigo/pull/100)