Add support for maxConcurrency parameter in SqsListener

[ngaya-ll]

See issue for background.

Fixes gh-379

[pivotal-issuemaster]

@ngaya-ll Please sign the Contributor License Agreement!

Click here to manually synchronize the status of this Pull Request.

See the FAQ for frequently asked questions.

[pivotal-issuemaster]

@ngaya-ll Thank you for signing the Contributor License Agreement!

[ngaya-ll]

Pinging @alainsahli, @aemruli as the primary authors of SimpleMessageListenerContainer.

[reachkrishnaraj]

It would be great to have each listener assign its own TaskExecutor with its own configs like concurrency, polling config etc.,

[mateuszmrozewski]

@ngaya-ll, could you resolve the conflicts, please? It's been a while since you prepared the PR, but maybe we can get the project owner attention and get that merged. This is a crucial improvement.

[nickcaballero]

Seems like this PR was abandoned but it has a lot of value. The other alternative is to use the JMS library from AWS which supports multiple listeners. @ngaya-ll, if you don't mind I can pick up where you left off with this work? Or maybe you can get the conflicts resolved and move this forward?