Writable connections are pinged at a slower rate.
The function IsPingable will filter out the writable connections.
The interval for slower ping rate by default is WRITABLE_CONNECTION_PING_INTERVAL(2500ms) and can be set with the configuration.
BUG=webrtc:1161
Review-Url: https://codereview.webrtc.org/1944003002
Cr-Commit-Position: refs/heads/master@{#12736}