Optional<UID> get(final Txn<ByteBuffer> txn, final MapDefinition mapDefinition) {
        return mapUidForwardDb.get(txn, mapDefinition);
    }
Returns the UID corresponding to the passed mapDefinition if it exists in the two mapping DBs. If it doesn't
exist, a forward and reverse mapping will be created and the new UID returned. The returned UID wraps a
direct allocation {@link ByteBuffer} owned by LMDB, so it may ONLY be used whilst still inside the passed
{@link Txn}.
stroom-pipeline/src/main/java/stroom/pipeline/refdata/store/offheapstore/MapDefinitionUIDStore.java