NodeFailureHistory: gather detection time alongside with durations.
Implementing GC in engine.
Almost implemented GC mechanism
NodesManager: rewritten from scratch.
NodeInfo: LastPingTimestamp gone internal.
NodesManager: tested failure history accumulation.
Master: collect garbage in TasksManager.
TaskState: ReportCompletion() returns result.
TaskManager: bugfix.
TaskManager: unit tests.
Master: a bit of logging.
FileBasedLog: thread-safe.