of_01: Coalesce connection aborted messages

A recent commit made these happen at debug level instead of info.  We now
coalesce them into chunks of 20 seconds too.  There's some information
loss here, but I think it's acceptable.
nicira: nx_output_reg can infer nbits

Other code could already do this.  Now this can too.
nicira: Minor improvements to nx_match

* Slightly better docstring
* Can do support "in" operator ("NXM in match")
nicira: nx_reg_load improvements

* Require setting .value
* .value can now be an nxm_entry with a value
discovery: Include causal event in LinkEvent

Sometimes an OpenFlow even caused a LinkEvent.  We now include it in the
LinkEvent so that listeners can examine it.
discovery: Minor refactoring

Creating an LLDP packet and creating a packet-out containing one are now
distinct operations.