Bug Description:
When reduce db.maxOpen via db.SetMaxOpenConns, the unnecssary
connections won't been released until all other connections are free.

Fixes #9453

Change-Id: I9afb2e4b184139b31029ae53d7f5fd1fdb8d8d7e
Reviewed-on: https://go-review.googlesource.com/2200


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>