database: Find the FeatureVersion we try to insert before doing any lock

This commit is issued in order to limit the bottleneck that the
exclusive database lock on Vulnerability_Affects_FeautreVersion
introduces, when we inserting FeatureVersions. This slowdowns a bit
the FeatureVersion insertion on a mostly empty database but should
increase a lot the throughput and parallelism on a populated database.

database/pgsql/feature.go

@@ -91,6 +91,23 @@ func (pgSQL *pgSQL) insertFeatureVersion(featureVersion database.FeatureVersion)
 
 	featureVersion.Feature.ID = featureID
 
+	// Try to find the FeatureVersion.
+	//
+	// In a populated database, the likelihood of the FeatureVersion already being there is high.
+	// If we can find it here, we then avoid using a transaction and locking the database.
+	err = pgSQL.QueryRow(searchFeatureVersion, featureID, &featureVersion.Version).
+		Scan(&featureVersion.ID)
+	if err != nil && err != sql.ErrNoRows {
+		return 0, handleError("searchFeatureVersion", err)
+	}
+	if err == nil {
+		if pgSQL.cache != nil {
+			pgSQL.cache.Add(cacheIndex, featureVersion.ID)
+		}
+
+		return featureVersion.ID, nil
+	}
+
 	// Begin transaction.
 	tx, err := pgSQL.Begin()
 	if err != nil {

database/pgsql/queries.go

@@ -52,6 +52,9 @@ const (
 		UNION
 		SELECT id FROM new_feature`
 
+	searchFeatureVersion = `
+		SELECT id FROM FeatureVersion WHERE feature_id = $1 AND version = $2`
+
 	soiFeatureVersion = `
 		WITH new_featureversion AS (
 			INSERT INTO FeatureVersion(feature_id, version)