INT-3087:Fix MongoDbMessageStore RemoveMessage Bug

Add new query so the same message found in pollMessageFromGroup is the
one which gets deleted.

See jira for more details: https://jira.springsource.org/browse/INT-3087

INT-3087 Polishing

Use an atomic method in pollMessageFromGroup to pop the first message.

Add test cases for pollMessageFromGroup and removeMessageFromGroup
where the same message exists in multiple groups.