JIRA: https://jira.springsource.org/browse/INT-3181 INT-3181: Fix JdbcChannelMS#pollMessageFromGroup * Add `doRemoveMessageFromGroup` and check its result. If it is `false` then just return `null`, not polled `Message` * Add `AbstractTxTimeoutMessageStoreTests#testInt3181ConcurrentPolling` test and implement it for some DBs * Make a note regarding MVCC as important in the Doc Minor Polishing