Add support for executing RPCs

Creating a temporary YangAccess for RPC input means I need to somehow
give the right libyang schemas. For that reason I supply a callable
which is able to fetch the schema and create a YangAccess instance for
ProxyDatastore.

The ProxyDatastore class now has a simple mechanism for deciding whether
to use the normal datastore and the temporary based on a path prefix.

Change-Id: Ib455f53237598bc2620161a44fb89c48ddfeb6e3
diff --git a/tests/example-schema.yang b/tests/example-schema.yang
index 792ac42..04ae22d 100644
--- a/tests/example-schema.yang
+++ b/tests/example-schema.yang
@@ -111,6 +111,16 @@
         }
     }
 
+    rpc fire {
+        input {
+            leaf whom {
+                type leafref {
+                    path '/aha:person/name';
+                }
+            }
+        }
+    }
+
     rpc launch-nukes {
         input {
             container payload {