Incorrect stream processing in takeover during rebalance. In-flight mutations may not persist correctly

Takeover is the name we give to the manoeuvre where we change the location of an active vbucket. The old active vbucket marks itself as dead, sends all outstanding mutations on the "takeover stream" to the new master and then sets the state of the new master to active. However, it appears a race can occur where not all of the outstanding mutations get sent to the new master resulting in the mutations not persisting correctly.