OFConnectionManager: drop more types of messages when the write buffer partially fills

Refactored to classify certain message types as "noncritical" and drop them
when reaching a high water mark.

Previously only OF_FLOW_REMOVED and OF_PACKET_IN would be dropped. This adds
OF_BSN_FLOW_IDLE, OF_BSN_ARP_IDLE, and OF_BSN_PDU_RX_TIMEOUT.
Merge into master from pull request #137:
OFConnectionManager: drop more types of messages when the write buffer partially fills (https://github.com/floodlight/indigo/pull/137)