This adds a new context.Context argument to the Backend.Configure method,
updates all of the implementations to match, and then updates all of the
callers to pass in a context.
A small number of callers don't yet have context plumbed to them so those
use context.TODO() as a placeholder for now, so we can more easily find
and fix them in later commits once we have contexts more thoroughly
plumbed.
Signed-off-by: Martin Atkins <mart@degeneration.co.uk>
