Allow message acknowledgment outside of the message delivery thread

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Several vendors currently support individual message acknowledgment outside of message listeners, that is, on a thread other than the thread of control of the Session. This is useful for certain types of application, for example, the asynchronous processing of queue or durable topic subscriber messages (for which individual message acknowledgment would also be useful, see #95).

The fact that several vendors already support this usage demonstrates the value of this feature.

It is therefore proposed to alter the JMS specification to explicitly allow message acknowledgment from a thread other than the message delivery thread of the Session that runs the MessageListeners. Currently the only Session-related methods which are allowed to be called outside of the thread of control of the Session are Session.close and Consumer.close (JMS 2.0 spec section 6.5). The proposed enhancement would add the Message.acknowledge method to this list when individual message acknowledge mode is being used (see #95).
#### Affected Versions
[1.1]

Review JMS 2.0 specification section 6.5 and related issue #95 first. Done means the specification explicitly addresses whether Message.acknowledge may be called from a non-delivery thread in individual acknowledgment mode.

Written by the indexing model from the issue text.