resources: Polish the password PoC and build out send/recv

This polishes the password resource so that it can actually avoid
writing the password to disk, and so that the work actually happens in
CheckApply where it can properly interact with the graph. This resource
now re-generates the password when it receives a notification.

The send/recv plumbing has been extended so that receivers can detect
when they're receiving new values. This is particularly important if
they might otherwise not expect those values to change and cache them
for efficiency purposes.

resources/refresh.go

@@ -19,6 +19,7 @@ package resources
 
 import (
 	"fmt"
+	"io/ioutil"
 	"os"
 	"strings"
 
@@ -68,12 +69,11 @@ func (obj *DiskBool) Get() (bool, error) {
 		return false, errwrap.Wrapf(err, "could not read token")
 	}
 	defer file.Close()
-	str := obj.str()
-	data := make([]byte, len(str)) // data + newline
-	if _, err := file.Read(data); err != nil {
+	data, err := ioutil.ReadAll(file)
+	if err != nil {
 		return false, errwrap.Wrapf(err, "could not read from file")
 	}
-	return strings.TrimSpace(string(data)) == strings.TrimSpace(str), nil
+	return strings.TrimSpace(string(data)) == strings.TrimSpace(obj.str()), nil
 }
 
 // Set stores the true boolean value, if no error setting the value occurs.