[FIXED JENKINS-28245] Allow finer-grained tuning of ChannelPinger.

 * Allows customization in seconds, not minutes
 * Allows customization of the ping timeout (before, you could set a
   custom interval, but the timeout would always be PingThread's 4
   minute default)

This also drops the serialVersionUID from ChannelPinger.SetUpRemotePing;
without one provided, the JVM will generate one on demand which is
sufficient for the purposes here since these are never persisted and
master & slave run the same compiled code. (And it demonstrably works
since countless other MasterToSlaveCallables fail to specify their own
custom IDs)