Rework CallProcedure to validate result sets

Summary:
This diff renames `__reload__` procedure to be `mg.reload` accepting a
module name. The main CallCustomProcedure function is now split into
multiple parts, so that there's more control over finding a procedure,
type checking its arguments and finally checking the returned result
set.

Depends on D2572

Reviewers: mferencevic, ipaljak

Reviewed By: ipaljak

Subscribers: pullbot

Differential Revision: https://phabricator.memgraph.io/D2573

query_modules/example.c

@@ -68,7 +68,8 @@ int mgp_init_module(struct mgp_module *module, struct mgp_memory *memory) {
   }
   mgp_value_destroy(null_value);
   if (!mgp_proc_add_result(proc, "result", mgp_type_string())) return 1;
-  if (!mgp_proc_add_result(proc, "args", mgp_type_list(mgp_type_any())))
+  if (!mgp_proc_add_result(proc, "args",
+                           mgp_type_list(mgp_type_nullable(mgp_type_any()))))
     return 1;
   return 0;
 }