ZOOKEEPER-1797. PurgeTxnLog may delete data logs during roll (Rakesh R via michim)


git-svn-id: https://svn.apache.org/repos/asf/zookeeper/branches/branch-3.4@1595553 13f79535-47bb-0310-9956-ffa450edef68
Merge remote-tracking branch 'remotes/origin/branch-3.4' into branch-3.4