arpa: create structs for arp table key and value
arpa: allocate private data for each entry
arpa: index entries with a hashtable
arpa: detect MAC mismatch and send to controller
arpa: maintain counters and idle time
arpa: parse timeout value TLVs
arpa: do not send all ARP replies to the controller

ARP replies should only be sent to the controller if the source check failed.
Merge into master from pull request #18:
ARP filtering and statistics (https://github.com/floodlight/switchlight-common/pull/18)