JCR-3483 nullify current node element before session is alive check is made

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/branches/2.4@1421477 13f79535-47bb-0310-9956-ffa450edef68
JCR-3486 guard against npe due to transient root being null

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/branches/2.4@1421854 13f79535-47bb-0310-9956-ffa450edef68
JCR-3469: Thread interrupt may result in closed index files
- merged into 2.4 branch

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