Ping automatically every 2 minutes (unconditionally)

... instead of after 30 minutes of no sending, for latency measurement
and keep-alive. Also, disconnect if no reply arrives within 20 minutes,
instead of 90 of inactivity (for peers supporting the 'pong' message).
Merge pull request #2784

f1920e8 Ping automatically every 2 minutes (unconditionally) (Pieter Wuille)