Add sqlx.Conn to correspond to sql.Conn

[andeya]

Requires version: go ≥ 1.9

[coveralls]

Pull Request Test Coverage Report for Build 30

• 0 of 58 (0.0%) changed or added relevant lines in 1 file are covered.
• 80 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-2.7%) to 69.147%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
sqlx_conn.go	0	58	0.0%
<!--	Total:	0	58

Files with Coverage Reduction	New Missed Lines	%
reflectx/reflect.go	80	97.67%
<!--	Total:	80

Totals
Change from base Build 17:	-2.7%
Covered Lines:	1078
Relevant Lines:	1559

---

💛 - Coveralls

[jmoiron]

Nice contribution. I'd like to add some testing and perhaps get vgo integration going before I leave some verisons of Go behind.

[andeya]

The test just like this:

type Stat struct {
	Uri       string `db:"uri"`
	Total     int64  `db:"total"`
	CreatedAt int64  `db:"created_at"`
}

func TestDBConn(t *testing.T) {
	conn, err := db.Conn(context.Background())
	if err != nil {
		t.Fatal(err)
	}
	defer conn.Close()
	var total int64 = 1
	r, err := conn.ExecContext(
		context.Background(),
		`INSERT INTO stat VALUE ('/j',?,1528996897)
		ON DUPLICATE KEY UPDATE total=@total_tmp:=total+?;`,
		total, 1,
	)
	if err != nil {
		t.Fatal(err)
	}
	rowsAffected, err := r.RowsAffected()
	if err != nil {
		t.Fatal(err)
	}
	if rowsAffected == 2 {
		err = conn.GetContext(context.Background(), &total, "SELECT @total_tmp as total;")
		if err != nil {
			t.Fatal(err)
		}
	}
	t.Log(total)
}

[glaslos]

@henrylee2cn how about adding your test to the PR?