Require access to ConnString via Config

A Config object may be created via ParseConfig and then further
modified. Requiring access to the original ConnString via the Config
helps indicate that the Config is the source of truth as to how the
connection was actually established.

pgxpool/pool_test.go

@@ -17,7 +17,7 @@ func TestConnect(t *testing.T) {
 	connString := os.Getenv("PGX_TEST_DATABASE")
 	pool, err := pgxpool.Connect(context.Background(), connString)
 	require.NoError(t, err)
-	assert.Equal(t, connString, pool.ConnString())
+	assert.Equal(t, connString, pool.Config().ConnString())
 	pool.Close()
 }