HBASE-10745 Access ShutdownHook#fsShutdownHooks should be synchronized

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HBASE-10549 When there is a hole, LoadIncrementalHFiles will hang in an infinite loop.(yuanxinen)

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HBASE-10514 Forward port HBASE-10466, possible data loss when failed flushes

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HBASE-19751 TestHRegion testWritesWhileScanning occasional fail since HBASE-10514 went in

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