HDFS-4482. ReplicationMonitor thread can exit with NPE due to the race between delete and replication of same file. Contributed by Uma Maheswara Rao G.

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Merge remote branch 'origin/branch-0.23' into branch-0.23-gd
Merge remote branch 'origin/branch-0.23-gd' into fb-YAHOO-226-branch-0.23-gd