Reduce searches for open sessions

[Arceliar]

Sort-of related to #65 (but for usage-based resource consumption instead of idle).

Currently, if no traffic has been received for a session in at least 6 seconds, then the next time we send a packet we also start a search. This is to quickly detect if a session has dropped due to coord changes.

What we should do is switch to sending sessionPings (throttled to a max of 1/second/session). If an additional T time (3-6 seconds?) passes without receiving a sessionPong, then try a search instead. Most of the time, a lack of traffic probably just means the session was idle, so a search is overkill.