INT-3276 Use a ThreadPool For Aggregator Stress

Stress tests previously created 36,000 threads.

JIRA: https://jira.springsource.org/browse/INT-3276
INT-3276 Failing CI Tests

* Add test diagnostics
* Increase timers

JIRA: https://jira.springsource.org/browse/INT-3276
Initial MQTT commit
INTEXT-64 - Polishing

* Update dependency versions
* Add more comments/links
* Remove DocBook usage
INTEXT-89 Fix Inbound Stop When No Server

The reconnection task was not canceled in doStop().
MQTT - Use INT/SPR 4.0.0
MQTT - Update to Spring Integration 4.0.0.M1
Updated MQTTMessageConverter to use new classpath
INT-3239 MQTT Final Polishing For Master Merge

* Final Polishing to move from extensions to master
* Commit history is retained from the extensions
* Docbook Chapter

JIRA: https://jira.springsource.org/browse/INT-3239
INT-3271 Revert MessageHandlingException Ctor Use

When `MHE` was moved to spring-messaging, 2 constructors were
dropped. INT-3246 worked around it but it had undesirable
side-effects. Constructors have been added back in
spring-messaging so this commit reverts to using those
constructors.

JIRA: https://jira.springsource.org/browse/INT-3271