database/sql: fix logic for pulling a Conn from DB

The logic for pulling a database connection from the DB pool should
proceed as follows: attempt to pull either a cached connection or
new connection N times in a loop. If each connection results
in a bad connection, then create a new connection (no cache).

Previously pulling a Conn from the pool, the last step also
looked at the cache, rather then always creating a new connection.

Fixes #29684

Change-Id: I8f436fd9b96eb35502a620ebe8da4ab89fb06a2e
Reviewed-on: https://go-review.googlesource.com/c/157637Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/database/sql/sql.go

@@ -1698,7 +1698,7 @@ func (db *DB) Conn(ctx context.Context) (*Conn, error) {
 		}
 	}
 	if err == driver.ErrBadConn {
-		dc, err = db.conn(ctx, cachedOrNewConn)
+		dc, err = db.conn(ctx, alwaysNewConn)
 	}
 	if err != nil {
 		return nil, err