runtime: allow cgo callbacks on non-Go threads

Fixes issue 4435.

R=golang-dev, iant, alex.brainman, minux.ma, dvyukov
CC=golang-dev
https://codereview.appspot.com/7304104
database/sql: refcounting and lifetime fixes

Simplifies the contract for Driver.Stmt.Close in
the process of fixing issue 3865.

Fixes issue 3865
Update issue 4459 (maybe fixes it; uninvestigated)

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7363043
image/png: use Scanner in reader_test.

R=nigeltao
CC=golang-dev
https://codereview.appspot.com/7399044
net/http: improve test reliability

Fixes issue 4852

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/7374045
go/types: export data result types are always parenthesized

Minor simplification of gcimporter, removed TODO.

R=adonovan
CC=golang-dev
https://codereview.appspot.com/7363044
misc/dashboard/builder: various cleanups

* allow commit watcher to be disabled, useful for small slow builders who will never be the first to notice a commit.
* builders always update their local master working copy before cloning a specific revision.
* refactor hg repo operations into a new type, Repo.

R=adg, shanemhansen, luitvd
CC=golang-dev
https://codereview.appspot.com/7326053
database/sql: clarify that DB.Prepare's stmt is safe for concurrent use

And add a test too, for Alex. :)

Fixes issue 3734

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/7399046