Convert actor dummy objects to task execution edges. (#1281)

* Define execution dependencies flatbuffer and add to Redis commands

* Convert TaskSpec to TaskExecutionSpec

* Add execution dependencies to Python bindings

* Submitting actor tasks uses execution dependency API instead of dummy argument

* Fix dependency getters and some cleanup for fetching missing dependencies

* C++ convention

* Make TaskExecutionSpec a C++ class

* Convert local scheduler to use TaskExecutionSpec class

* Convert some pointers to references

* Finish conversion to TaskExecutionSpec class

* fix

* Fix

* Fix memory errors?

* Cast flatbuffers GetSize to size_t

* Fixes

* add more retries in global scheduler unit test

* fix linting and cast fbb.GetSize to size_t

* Style and doc

* Fix linting and simplify from_flatbuf.

src/common/lib/python/common_extension.h

@@ -1,6 +1,8 @@
 #ifndef COMMON_EXTENSION_H
 #define COMMON_EXTENSION_H
 
+#include <vector>
+
 #include <Python.h>
 #include "marshal.h"
 #include "structmember.h"
@@ -22,6 +24,7 @@ typedef struct {
   PyObject_HEAD
   int64_t size;
   TaskSpec *spec;
+  std::vector<ObjectID> *execution_dependencies;
 } PyTask;
 // clang-format on