Use single callback for replies to async operations

Single callback simplifies copying of callbacks and makes
control flow more obvious.

BUG:24267885

Change-Id: I489e7158e2bb1adf8c9c3966a0859fa024a57db2
Reviewed-on: https://weave-review.googlesource.com/1302
Reviewed-by: Vitaly Buka <vitalybuka@google.com>