add ConcurrentUtil.newScheduledThreadPoolExecutor

Use helper factory method to create ScheduledThreadPoolExecutors
(related to jPOS-102 (https://jpos.org/issues/issue/jPOS-102)
added helper getScheduledThreadPoolExecutor
jPOS-102 use ScheduledThreadPoolExecutor

use ScheduledThreadPoolExecutor instead of Timer
This makes the code more readable, specially in the work-around
a race condition where a response could arrive exactly at expiration
time, code is much more readable now using future.cancel.

Removed unused variable and jmx-doc stuff we are not using anymore.