Lock getLockForKey(final Object key) {
        int lockNumber = selectLock(key, DEFAULT_NUMBER_OF_MUTEXES);
        return mutexes[lockNumber];
    }
We use a number here based on our typical concurrency profile. So 10
indexing threads and 2048 locks seems OK.
stroom-docstore/stroom-docstore-impl-fs/src/main/java/stroom/docstore/impl/fs/StripedLock.java