database/sql: allow simultaneous queries, etc in a Tx

Now that revision 0c029965805f is in, it's easy
to guarantee that we never access a driver.Conn
concurrently, per the database/sql/driver contract,
so we can remove this overlarge mutex.

Fixes #3857

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/7707047

src/pkg/database/sql/sql_test.go

@@ -736,3 +736,28 @@ func TestIssue4902(t *testing.T) {
 		t.Logf("stmt = %#v", stmt)
 	}
 }
+
+// Issue 3857
+// This used to deadlock.
+func TestSimultaneousQueries(t *testing.T) {
+	db := newTestDB(t, "people")
+	defer closeDB(t, db)
+
+	tx, err := db.Begin()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer tx.Rollback()
+
+	r1, err := tx.Query("SELECT|people|name|")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer r1.Close()
+
+	r2, err := tx.Query("SELECT|people|name|")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer r2.Close()
+}