Allow ActivityWorker to shutdown cleanly

ActivityWorker doesn't shutdown cleanly.

It registers a @shutdown_first_time_function in initialize that does a Kernel.exit on sigint/term.  But the exit exception is caught by the poller (task_poller.rb line 182), so the ActivityWorker continues the task polling loop.

This change just ends the loop if the worker is shutting down.
ActivityTaskPoller quits polling early

The ActivityTaskPoller fails to check for poll timeouts before logging information about the task.  As a result, it throws an exception, which is caught by the ActivityWorker, which just retries.  There's no real harm done, since the ActivityWorker retries the failed poll, but this cleans up the invalid log messages and lets the code behave as intended.
ForkingExecutor doesn't use provided logger config

Simple cleanup..
Make ActivityWorker shutdown consistent with WorkflowWorker shutdown
Merge branch 'master' of https://github.com/YesVideo/aws-flow-ruby into master