Breaks build, and has a race.

««« original CL description
database/sql: add SetMaxOpenConns

Update #4805

Add the ability to set an open connection limit.
Fixed case where the Conn finalCloser was being called with db.mu locked.
Added seperate benchmarks for each path for Exec and Query.
Replaced slice based idle pool with list based idle pool.

R=bradfitz
CC=golang-dev
https://golang.org/cl/10726044

»»»

R=golang-dev
CC=golang-dev
https://golang.org/cl/13252046