Individual message acknowledge mode

Currently the JMS 1.1 and 2.0 specifications only allow for one mode of operation for applications which wish to do their own message acknowledgment. This is CLIENT_ACKNOWLEDGE mode. This mode has the following characteristic, as defined in the spec: "Acknowledging a consumed message automatically acknowledges the receipt of all messages that have been delivered by its session.".

This Implicit acknowledgment of all messages is not only somewhat unexpected, because it's such a broad side effect, it is highly inconvenient in cases where message processing is being done by multiple threads asynchronously (message reception on one Session has to be in a single thread, per JMS spec rules, but I am talking about cases where the messages are then dispatched to one or more worker threads, or even other processes, for asynchronous processing). I realise this can be overcome to some degree by using a separate Session and MessageConsumer in each worker thread, but that imposes more overhead on the JMS provider. A better alternative, which is already offered by certain JMS providers (including Tibco EMS and ActiveMQ), is INDIVIDUAL_ACKNOWLEDGE mode, where acknowledging a message acknowledges only that message. This makes asynchronous processing of messages much easier to implement.

One can imagine other acknowledge modes that would be useful too, for example: CONSUMER_ACKNOWLEDGE where Message.acknowledge() would acknowledge only messages received up on a particular MessageConsumer, or CONSUMER_CHECKPOINT_ACKNOWLEDGE where Message.acknowledge() would acknowledge only messages received up to and including the Message instance on which the method was called.

But without embarking on all these various different possibilities, would it be possible to consider just adding INDIVIDUAL_ACKNOWLEDGE mode? This alone would make it possible for multithreaded applications to achieve whatever behaviors they need.
#### Environment
n/a
#### Affected Versions
[1.1]

Start by reviewing the JMS 1.1 and 2.0 acknowledgment semantics described in the issue, then read the existing discussion and proposals around INDIVIDUAL_ACKNOWLEDGE. Done means the requested acknowledgment mode is designed and specified clearly enough to support multithreaded asynchronous processing, with its behavior and affected versions addressed.

Written by the indexing model from the issue text.