QPID-5450: have the group manager try to acquire the message at the time it is accepted into the group, so that the associated shared-group state change occurs within the single synchronization block

merged from trunk r1556096


git-svn-id: https://svn.apache.org/repos/asf/qpid/branches/0.26@1556501 13f79535-47bb-0310-9956-ffa450edef68