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

Reviewer: @harshsin

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.