JBPM-3814, BZ 1002724 Bugfixes with regards to concurrency

- Changed EnvironmentImpl to use a concurrent HashMap
- The most significant persistence changes are in the JpaPersistenceContextManager class
- Reverted locking changes to the SingleSessionCommandService since a fix for JBPM-3814 is not
  possible at the level of the SSCS in the form of locking (because of the app-scoped entity
  manager).
- Also deleted unsused interfaces (ApplicationScopedPersistenceContext, TransactionablePersistentContext)