storage/sql: add a SQL storage implementation

This change adds support for SQLite3, and Postgres.

storage/sql/migrate_test.go

@@ -0,0 +1,25 @@
+package sql
+
+import (
+	"database/sql"
+	"testing"
+)
+
+func TestMigrate(t *testing.T) {
+	db, err := sql.Open("sqlite3", ":memory:")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer db.Close()
+
+	c := &conn{db, flavorSQLite3}
+	for _, want := range []int{len(migrations), 0} {
+		got, err := c.migrate()
+		if err != nil {
+			t.Fatal(err)
+		}
+		if got != want {
+			t.Errorf("expected %d migrations, got %d", want, got)
+		}
+	}
+}