queue: Take advantage of ABC for defining abstract worker base classes.

QueueProcessingWorker and LoopQueueProcessingWorker are abstract classes
meant to be subclassed by a class that will define its own consume()
or consume_batch() method. ABCs are suited for that and we can tag
consume/consume_batch with the @abstractmethod wrapper which will
prevent subclasses that don't define these methods properly to be
impossible to even instantiate (as opposed to only crashing once
consume() is called). It's also nicely detected by mypy, which will
throw errors such as this on invalid use:

error: Only concrete class can be given where "Type[TestWorker]" is
expected
error: Cannot instantiate abstract class 'TestWorker' with abstract
attribute 'consume'

Due to it being detected by mypy, we can remove the test
test_worker_noconsume which just tested the old version of this -
raising an exception when the unimplemented consume() gets called. Now
it can be handled already on the linter level.

zerver/worker/queue_processors.py

@@ -1,4 +1,5 @@
 # Documented in https://zulip.readthedocs.io/en/latest/subsystems/queuing.html
+from abc import ABC, abstractmethod
 from typing import Any, Callable, Dict, List, Mapping, Optional, cast, Tuple, TypeVar, Type
 
 import copy
@@ -119,7 +120,7 @@ def retry_send_email_failures(
 
     return wrapper
 
-class QueueProcessingWorker:
+class QueueProcessingWorker(ABC):
     queue_name = None  # type: str
 
     def __init__(self) -> None:
@@ -127,8 +128,9 @@ class QueueProcessingWorker:
         if self.queue_name is None:
             raise WorkerDeclarationException("Queue worker declared without queue_name")
 
+    @abstractmethod
     def consume(self, data: Dict[str, Any]) -> None:
-        raise WorkerDeclarationException("No consumer defined!")
+        pass
 
     def consume_wrapper(self, data: Dict[str, Any]) -> None:
         try:
@@ -184,8 +186,9 @@ class LoopQueueProcessingWorker(QueueProcessingWorker):
             if not self.sleep_only_if_empty or len(events) == 0:
                 time.sleep(self.sleep_delay)
 
+    @abstractmethod
     def consume_batch(self, events: List[Dict[str, Any]]) -> None:
-        raise NotImplementedError
+        pass
 
     def consume(self, event: Dict[str, Any]) -> None:
         """In LoopQueueProcessingWorker, consume is used just for automated tests"""