add bsn_log extension

Reviewer: @andi-bigswitch

This provides a mechanism for either the controller or switch to request the 
other side to output a log message, subject to the configured loglevel. This
is useful for log (and OpenFlow pcap) correlation.
I added a comment about the string encoding. Besides being variable length it's the same as any other string on the wire, so not sure if we need a new type for it. At least, the C and Python backends don't need it.