Introduce EventExecutor.shutdownGracefully() that deprecates shutdown()

shutdownGracefully() provides two optional parameters that give more
control over when an executor has to be shut down.

- Related issue: #1307
- Add shutdownGracefully(..) and isShuttingDown()
- Deprecate shutdown() / shutdownNow()
- Replace lastAccessTime with lastExecutionTime and update it after task
  execution for accurate quiet period check
  - runAllTasks() and runShutdownTasks() update it automatically.
  - Add updateLastExecutionTime() so that subclasses can update it
- Add a constructor parameter that tells not to add an unncessary wakeup
  task in execute() if addTask() wakes up the executor thread
  automatically.  Previously, execute() always called wakeup() after
  addTask(), which often caused an extra dummy task in the task queue.
- Use shutdownGracefully() wherever possible / Deprecation javadoc
- Reduce the running time of SingleThreadEventLoopTest from 40s to 15s
  using custom graceful shutdown parameters

- Other changes made along with this commit:
  - takeTask() does not throw InterruptedException anymore.
    - Returns null on interruption or wakeup
  - Make sure runShutdownTasks() return true even if an exception was
    raised while running the shutdown tasks
  - Remove unnecessary isShutdown() checks
  - Consistent use of SingleThreadEventExecutor.nanoTime()

Replace isWakeupOverridden with a constructor parameter