This creates a `Monitor` type that keeps the global state between
monitor invocations and allows the monitor to exist during the build so
it can be utilized for callbacks.
The result handler is now registered with the monitor during the build
and `Run` will use the result if it is present and the configuration
intends the monitor to be invoked with the given result.
Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>
Jonathan A. Sternberg
8f2604b6b4