- introduced directed message types.
- introduced toQueue, toTopic, to, and withoutDestination methods to
  switch between directed and regular messages.
- remove parens of properties() member in JmsMessage.
- added missing type annotations, removed unnecessary parens.
- improved scaladocs of message types.

- Divided JmsConnector trait into producer and consumer connector traits
  to simplify logic of each connector
- Divided consumer and produce sessions for the same reason
- Destination objects are cached in a memory-aware way
- Added tests and documentation of new flow

- As the producer is passed from one thread to the next only when the
  first one has completed its send() operation, it is sufficient to
  make writes visible across threads. No real synchronization is
  necessary.
- Along the way, harmonized javax.jms imports in JmsConnector to
  consistently use the jmx. prefix.

Session creation was asynchronous for consumers, but all sessions were
created in one thread. Producer sessions were created synchronously.
To reduce latency, session creation is fanned out to multiple threads.

- Added a comment to explain why the stage might pull when a new session
  is added
- Removed unnecessary check from in.onPush: we know a producer is
  available, since we only pull if the producer pool is non-empty

- Introduce optional Direction field on JmsMessage
- Fallback to default destination if no direction is defined on message

Fixes akka#1166

- reduced flakiness of session initialization
- aligned session creation between producer and consumer
- made JmsSession sealed
- simplified synchronization in SoftReferenceCache

Provided a test in SoftReferenceCacheSpec giving some evidence
(which of course is no proof) that synchronization is not needed for how
the cache is intended to be used.

SoftReferenceCache synchronization can piggyback on Akka's async
callback synchronization guarantess that is ultimately relying on
AtomicReference operations.