coalescing_context: avoid goroutine when possible
coalescing_context: avoid goroutine when possible

We don't need a goroutine until either something has either accessed
the done channel, or is adding a second context.  This avoids having a
goroutine for every outstanding block request, even when the ones that
aren't actively being fetched.

Issue: HOTPOT-2268

keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client