database/sql: improve standard deviation response time under high concurrency

See https://github.com/raggi/go-and-java for runtime benchmark.

The patch reduces the amount of map key search, moving connection oriented
variables onto the connection structs.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/8092045

Committer: Brad Fitzpatrick <bradfitz@golang.org>