public long getEntryCount(final Txn<ByteBuffer> txn) {
        return valueStoreMetaDb.getEntryCount(txn);
    }
Either gets the {@link ValueStoreKey} corresponding to the passed refDataValue
from the database if we already hold it or creates the entry in the database
and returns the generated key. To determine if we already hold it, the value
will be hashed and that hash will be looked up and any matches tested for equality
using the serialised bytes.
<p>
onExistingValueAction Action to perform when the value is found to already exist

@return A clone of the {@link ByteBuffer} containing the database key.
stroom-pipeline/src/main/java/stroom/pipeline/refdata/store/offheapstore/ValueStore.java