database/sql: de-duplicate various methods

database/sql: de-duplicate various methods

Form a new method pattern where *driverConn and
release functions are passed into the method.
They are named DB.execDC, DB.queryDC, DB.beginDC. This
allows more code to be de-duplicated when starting
queries.

The Stmt creation and management code are untouched.

Change-Id: I24c853531e511d8a4bc1f53dd4dbdf968763b4e7
Reviewed-on: https://go-review.googlesource.com/39630
Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>