Cleanup and simplification in the taskmanager and the local instances and mini cluster.
Fixed log levels, code cleanups and restore the thread interruption states for JobManager and TaskManager
Fixed heartbeat thread and startup/shutdown races in TaskManager
Fix concurrency bugs in cluster manager and cluster manager tests.
Simplifications of instance loading at startup.
Clean up comments and fix doc errors
Adjust iteration low level tests for reduced memory footprint
Improve error logging on RPC server.
Adjust tests LocalDistributedExecutor for low memory settings on build servers.
Post-merge fixes.