database/sql: allow drivers to only implement Context variants
Drivers shouldn't need to implement both Queryer and QueryerContext,
they should just implement QueryerContext. Same with Execer and
ExecerContext. This CL tests for QueryContext and ExecerContext
first so drivers do not need to implement Queryer and Execer
with an empty definition.
Fixes #21663
Change-Id: Ifbaa71da669f4bc60f8da8c41a04a4afed699a9f
Reviewed-on: https://go-review.googlesource.com/65733 Reviewed-by: Ian Lance Taylor <iant@golang.org>