public OptionalInt getReferenceCount(final Txn<ByteBuffer> txn, final ByteBuffer keyBuffer) {
        Optional<ByteBuffer> optValueBuffer = getAsBytes(txn, keyBuffer);

        return optValueBuffer
                .map(byteBuffer ->
                        OptionalInt.of(valueSerde.extractReferenceCount(byteBuffer)))
                .orElseGet(OptionalInt::empty);
    }
This store holds meta data about the corresponding entries in {@link ValueStoreDb}.
It shares the same key as {@link ValueStoreDb} and each entry in here has a corresponding
entry in {@link ValueStoreDb}.
<p>
The type part of the value defines the data type of the value in {@link ValueStoreDb}.
The referenceCount part is used to keep track of the number of key/value or range/value
entries that are associated with an entry in {@link ValueStoreDb}.
<p>
key        | value
(hash|id)  | (type|referenceCount)
---------------------------------------------
(1234|00)  | (0|001)
(1234|01)  | (0|001)
(4567|00)  | (0|001)
(7890|00)  | (1|001)
stroom-pipeline/src/main/java/stroom/pipeline/refdata/store/offheapstore/databases/ValueStoreMetaDb.java