Ion and Philipp's table retries (#10)

* Ion and Philipp's table retries

* Refactor the retry struct:
- Rename it from retry_struct to retry_info
- Retry information contains the failure callback, not the retry callback
- All functions take in retry information as an arg instead of its expanded fields

* Rename cb -> callback

* Remove prints

* Fix compiler warnings

* Change some CHECKs to greatest ASSERTs

* Key outstanding callbacks hash table with timer ID instead of callback data pointer

* Use the new retry API for table commands

* Memory cleanup in plasma unit tests

* fix Robert's comments

* add valgrind for common

src/common/test/example_task.h

@@ -11,4 +11,13 @@ task_spec *example_task(void) {
   return task;
 }
 
+task_instance *example_task_instance(void) {
+  task_iid iid = globally_unique_id();
+  task_spec *spec = example_task();
+  task_instance *instance =
+      make_task_instance(iid, spec, TASK_STATUS_WAITING, NIL_ID);
+  free_task_spec(spec);
+  return instance;
+}
+
 #endif