JBPM-3814, BZ 1002724 Session thread safe support in JTA/EE environment (cherry picked from commit 6699bffb189a8c0009740ed7818575332c0ed533)

- Added ReentrantLock lock field to SSCS to take care of this
- Removed unneccessary perssimistic locking code (taken care of at the transaction manager level)
- Moved isSpringTransactionManager(..) method up from bottom to after initTransactionManager(..) method