Better and more sensible executor configuration

Dan and I had a chat about ISPN-1396 and we both agreed that a better configuration and management approach is needed for Infinispan's executors:

• Firstly, out of the box for SE environments, Infinispan should have executors configured with newCachedThreadPool because they provide better queuing performance than a fixed thread pool.

• Sure, in an managed env (i.e. AS), this won't fly, which is why all executors need to be injectable. This should be in place once ISPN-1396 is in place.

• So, if we go for cached thread pools for SE environments, we don't need any of the properties of executors any more. Besides, these can be more confusing for the user (there is knowledged of at least one case where things went wrong due to bad config here). So, the configuration would be limited to injecting executors. If you need any specific executor settings, pass us the right executors. To aid these cases, we could have some executor builders available with some common executor configuration for managed envs (i.e. we could borrow settings from AS?)