Replace further work by Scheduler class based on the "sched" module.

To support multiple backend, we need to get rid of `further_work` concept
which is used in multiple places in the MBS code. Before this commit, if
one handler wanted to execute another handler, it planned this work by
constructing fake message and returning it. MBSConsumer then planned
its execution by adding it into the event loop.

In this commit, the new `events.scheduler` instance of new Scheduler
class is used to acomplish this. If handler wants to execute another
handler, it simply schedules it using `events.scheduler.add` method.

In the end of each handler, the `events.scheduler.run` method is
executed which calls all the scheduled handlers.

The idea is that when Celery is enabled, we can change the
`Scheduler.run` method to execute the handlers using the Celery, while
during the local builds, we could execute them directly without Celery.

Use of Scheduler also fixes the issue with ordering of such calls. If
we would call the handlers directly, they could have been executed
in the middle of another handler leading to behavior incompatible
with the current `further_work` concept. Using the Scheduler, these
calls are executed always in the end of the handler no matter when
they have been scheduled.

module_build_service/builder/MockModuleBuilder.py

@@ -318,6 +318,7 @@ class MockModuleBuilder(GenericBuilder):
         pass
 
     def buildroot_add_artifacts(self, artifacts, install=False):
+        from module_build_service.scheduler.handlers.repos import done as repos_done_handler
         self._createrepo()
 
         # TODO: This is just hack to install module-build-macros into the
@@ -330,9 +331,7 @@ class MockModuleBuilder(GenericBuilder):
                 self.groups.append("module-build-macros")
                 self._write_mock_config()
 
-        from module_build_service.scheduler.consumer import fake_repo_done_message
-
-        fake_repo_done_message(self.tag_name)
+        events.scheduler.add(repos_done_handler, ("fake_msg", self.tag_name + "-build"))
 
     def tag_artifacts(self, artifacts):
         pass
@@ -394,6 +393,8 @@ class MockModuleBuilder(GenericBuilder):
         self._write_mock_config()
 
     def _send_build_change(self, state, source, build_id):
+        from module_build_service.scheduler.handlers.components import (
+            build_task_finalize as build_task_finalize_handler)
         try:
             nvr = kobo.rpmlib.parse_nvr(source)
         except ValueError:
@@ -401,18 +402,10 @@ class MockModuleBuilder(GenericBuilder):
 
         # build_id=1 and task_id=1 are OK here, because we are building just
         # one RPM at the time.
-        module_build_service.scheduler.consumer.work_queue_put({
-            "msg_id": "a faked internal message",
-            "event": events.KOJI_BUILD_CHANGE,
-            "build_id": build_id,
-            "task_id": build_id,
-            "build_name": nvr["name"],
-            "build_new_state": state,
-            "build_release": nvr["release"],
-            "build_version": nvr["version"],
-            "module_build_id": None,
-            "state_reason": None
-        })
+        args = (
+            "a faked internal message", build_id, build_id, state, nvr["name"], nvr["version"],
+            nvr["release"], None, None)
+        events.scheduler.add(build_task_finalize_handler, args)
 
     def _save_log(self, resultsdir, log_name, artifact_name):
         old_log = os.path.join(resultsdir, log_name)