[release-branch.go1.8] database/sql: do not exhaust connection pool on conn request timeout

Previously if a context was canceled while it was waiting for a
connection request, that connection request would leak.

To prevent this remove the pending connection request if the
context is canceled and ensure no connection has been sent on the channel.
This requires a change to how the connection requests are represented in the DB.

Fixes #18995

Change-Id: I9a274b48b8f4f7ca46cdee166faa38f56d030852
Reviewed-on: https://go-review.googlesource.com/36563


Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-on: https://go-review.googlesource.com/36613


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