resources: rate limiting: Implement resource rate limiting

This adds rate limiting with the limit and burst meta parameters. The
limits apply to how often the Process check is called. As a result, it
might get called more often than there are Watch events due to possible
Poke/BackPoke events.

This system might need to get rethought in the future depending on its
usefulness.

examples/file4.yaml

@@ -0,0 +1,13 @@
+---
+graph: mygraph
+resources:
+  file:
+  - name: file1
+    path: "/tmp/mgmt/f1"
+    meta:
+      limit: 0.5
+      burst: 3
+    content: |
+      i am f1
+    state: exists
+edges: []