ISPN-3448 Tests from org.infinispan.distribution.rehash package fail randomly

* NonTxStateTransferOverwritingValue2Test should block rebalance
  confirmations to avoid the retry of the user's put command.
* NonTxJoinerBecomingBackupOwnerTest should wait until the old owners
  collect the entries to send to the backup before doing the write,
  to avoid duplicate put commands on the joiner.