move execution delay detection to metrics collection stage, out of the hot path
fix concurrent modification bug in ConcurrentErrorReporter
make bounded queue size for OperationHandlerExecutors configurable
deprecate UniformWindow...StreamExecutor (temporary) to make it clear it is not used at present <-- will be reintroduced later for distributed execution
change Spinner return type from boolean to enum(PASSED, FAILED, STILL_WAITING), to capture more states