Add JMS defined property JMSXGroupLast

When grouping messages, there is no specified way to flag a message as the last one in a group.
To determine the last message in a group, one has to fall back to a vendor-specific property.
A good example can be found here: [http://www.ibm.com/developerworks/websphere/library/techarticles/0602_currie/0602_currie.html](http://www.ibm.com/developerworks/websphere/library/techarticles/0602_currie/0602_currie.html)

Adding a boolean JMS defined property with a name like JMSXGroupLast or similar would enable receiving group messages to be portable among different JMS providers.
#### Affected Versions
[2.0]

Start by reviewing the JMS 2.0 message-grouping and defined-property specifications, then compare the linked IBM example with the portability gap described here. Determine whether a boolean JMSXGroupLast-style property fits the standard and clarify its semantics; done means the specification has an agreed portable definition and any required coverage is identified.

Written by the indexing model from the issue text.