ClosedChannelException in flush()

[bfulton]

Apparent race during flush:

2017-11-16 16:19:03,014 [auto-flush-exec-pool-16] INFO  com.upserve.uppend.FileAppendOnlyStore - flushed data/my/store/A
2017-11-16 16:19:03,040 [auto-flush-exec-pool-20] ERROR com.upserve.uppend.lookup.LongLookup - unable to flush data/my/store/B/lookups/0/12/106
java.nio.channels.ClosedChannelException: null
    at sun.nio.ch.FileChannelImpl.ensureOpen(FileChannelImpl.java:110)
    at sun.nio.ch.FileChannelImpl.force(FileChannelImpl.java:379)
    at com.upserve.uppend.lookup.LookupData.flush(LookupData.java:202)
    at com.upserve.uppend.lookup.LongLookup.lambda$null$7(LongLookup.java:228)