accounts: keep the full *db.BaseDB pointer

This will make it possible in SQL unit tests to extract the BaseDB used
for accounts and use it to init another Store backed by the same BaseDB.

accounts/store_sql.go

@@ -67,8 +67,8 @@ type SQLStore struct {
 	// in order to implement all its CRUD logic.
 	db BatchedSQLQueries
 
-	// DB represents the underlying database connection.
-	*sql.DB
+	// BaseDB represents the underlying database connection.
+	*db.BaseDB
 
 	clock clock.Clock
 }
@@ -83,9 +83,9 @@ func NewSQLStore(sqlDB *db.BaseDB, clock clock.Clock) *SQLStore {
 	)
 
 	return &SQLStore{
-		db:    executor,
-		DB:    sqlDB.DB,
-		clock: clock,
+		db:     executor,
+		BaseDB: sqlDB,
+		clock:  clock,
 	}
 }