reduce pings

A PING is to prevent the broker from disconnecting due to inactivity.

Currently the pinger 1) starts right away with sending PINGREQs and 2) sends again after keep-alive seconds whether there was activity or not during this period.

The first creates unnecessary activity on a client that's 'booting'. The latter reduces packets sent (but is less of a win).

proposal

v0

when starting the pinger start with sleeping instead of sending a packet.