MULE-7731 JMS transport should reuse javax.jms.Session, javax.jms.Messag...

...eProducer instances by default
I suspect the tests were slower because they were using the default of 1 cached session which adds overhead but not caching.  I'd been interested to see results if they are still available though.

While there isn't a change in user behaviuor here, I agree that it is a good idea to have a mechanism to revert to 3.5 behaviuor just in case.

IMO this isn't something we need to expose to users and would suggest a system property here, but given <jms:connector/> already has an attribute to enable session caching (I hadn't seen this before) I agree it makes sense to use this with a default value of true.  If we think it makes sense we can deprecate this attribute in the future.