change: smarter change notifications

This commit replaces the ChangeSet with a simpler bool based
change model that can be directly used in the map builder to
build the appropriate map response based on the change that
has occured. Previously, we fell back to sending full maps
for a lot of changes as that was consider "the safe" thing to
do to ensure no updates were missed.

This was slightly problematic as a node that already has a list
of peers will only do full replacement of the peers if the list
is non-empty, meaning that it was not possible to remove all
nodes (if for example policy changed).

Now we will keep track of last seen nodes, so we can send remove
ids, but also we are much smarter on how we send smaller, partial
maps when needed.

Fixes #2389

Signed-off-by: Kristoffer Dalby <kristoffer@dalby.cc>

hscontrol/mapper/builder.go

@@ -29,10 +29,8 @@ type debugType string
 const (
 	fullResponseDebug   debugType = "full"
 	selfResponseDebug   debugType = "self"
-	patchResponseDebug  debugType = "patch"
-	removeResponseDebug debugType = "remove"
 	changeResponseDebug debugType = "change"
-	derpResponseDebug   debugType = "derp"
+	policyResponseDebug debugType = "policy"
 )
 
 // NewMapResponseBuilder creates a new builder with basic fields set.