database/sql: add OpenDB to directly create a *DB without a DSN.
The current Open method limits the ability for driver maintainers
to expose options for their drivers by forcing all the configuration
to pass through the DSN in order to create a *DB.
This CL allows driver maintainers to write their own initialization
functions that return a *DB making configuration of the underlying
drivers easier.
Fixes #20268
Change-Id: Ib10b794f36a201bbb92c23999c8351815d38eedb
Reviewed-on: https://go-review.googlesource.com/53430 Reviewed-by: Daniel Theophanes <kardianos@gmail.com>
Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>