Fix for SQSSessionCallbackScheduler zombie threads

[yeshodhan]

After checking for session.isRunning() and adding a timeout for callbackQueue.wait(), I was able to achieve the results as below:

I'm not sure if the timeout needs to be configured somewhere ?

Easiest way to reproduce the issue is to disconnect from internet and run the spring DefaultMessageListenerContainer.

[tamershahin]

any news on this?

[volkanto]

can this be related to https://github.com/awslabs/amazon-sqs-java-messaging-lib/pull/108?

[ziyanli-amazon]

@yeshodhan I took over this library and recently doing some clean-up. This PR LGTM, can you do a git pull and push another commit? And how to interpret the graph you posted?