{@code HeartBeatInvoker} is responsible for sendingheartbeats to {@code ClusterManager} on behalf of a {@code GridNode}.
Once started, {@code HeartBeatInvoker} sends heart beats to the{@code ClusterHeartBeatService} on each {@code ClusterHeartBeatService.BEAT_PERIOD}.
@author Yohan Liyanage
@version 1.0
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