arpa: validate timeouts
arpa: implement timers

This is a simple and slow implementation. The ARP agent loops over every entry
once per second to see which have expired. This will be rewritten to be more
scalable later.
arpa: send unicast/broadcast queries on timeout
update loxigen-artifacts

To get bsn_arp_idle.

* submodules/loxigen-artifacts df3e83b...148e1f6 (5):
  > Artifacts from https://github.com/floodlight/loxigen/compare/ee486e9db76d2bfc56808fe53be5a6db943b90cc...ded3517401b92233c393ee8b082fa54fa68b18f4
  > Artifacts from https://github.com/floodlight/loxigen/compare/6e92d6892ca13a65a31711f9aacd8da9e7d82ce5...ee486e9db76d2bfc56808fe53be5a6db943b90cc
  > Artifacts from https://github.com/floodlight/loxigen/compare/960fb527bbe2c4f5cbf02159c74b5ef7ff58fbe6...6e92d6892ca13a65a31711f9aacd8da9e7d82ce5
  > Artifacts from https://github.com/floodlight/loxigen/compare/074f7096e8fd4e7a63d2b9cbba68e60be9a87f31...960fb527bbe2c4f5cbf02159c74b5ef7ff58fbe6
  > Artifacts from https://github.com/floodlight/loxigen/compare/f8498dd3102a8e03596d821c1b1c9ca80e4145d7...074f7096e8fd4e7a63d2b9cbba68e60be9a87f31
arpa: send idle notification message
arpa: demote requery/idle log messages to verbose
arpa: add trace log for timer state transitions
Merge into master from pull request #19:
arpa: unicast requery, broadcast requery, and idle timeouts (https://github.com/floodlight/switchlight-common/pull/19)