engine: Rewrite the core algorithm

The engine core had some unfortunate bugs that were the result of some
early design errors when I wasn't as familiar with channels. I've
finally rewritten most of the bad parts, and I think it's much more
logical and stable now.

This also simplifies the resource API, since more of the work is done
completely in the engine, and hidden from view.

Lastly, this adds a few new metaparameters and associated code.

There are still some open problems left to solve, but hopefully this
brings us one step closer.

engine/resources.go

@@ -21,8 +21,6 @@ import (
 	"encoding/gob"
 	"fmt"
 
-	"github.com/purpleidea/mgmt/engine/event"
-
 	errwrap "github.com/pkg/errors"
 	"gopkg.in/yaml.v2"
 )
@@ -93,22 +91,14 @@ type Init struct {
 	// Called from within Watch:
 
 	// Running must be called after your watches are all started and ready.
-	Running func() error
+	Running func()
 
 	// Event sends an event notifying the engine of a possible state change.
-	Event func() error
+	Event func()
 
-	// Events returns a channel that we must watch for messages from the
-	// engine. When it closes, this is a signal to shutdown.
-	Events chan *event.Msg
-
-	// Read processes messages that come in from the Events channel. It is a
-	// helper method that knows how to handle the pause mechanism correctly.
-	Read func(*event.Msg) error
-
-	// Dirty marks the resource state as dirty. This signals to the engine
-	// that CheckApply will have some work to do in order to converge it.
-	Dirty func()
+	// Done returns a channel that will close to signal to us that it's time
+	// for us to shutdown.
+	Done chan struct{}
 
 	// Called from within CheckApply: