WS: Change send ping logic
WS: Change send ping logic

Send ping frequently is required to avoid server initiated disconnection.
This patch changes the logic that manages WS ping and computes elapsed
time between each interval.

CESARBR/knot-service-source,CESARBR/knot-service-source,CESARBR/knot-service-source,CESARBR/knot-service-source,CESARBR/knot-service-source