Add serialization registry

[sevdog]

This PR aims at two goals:

1. Provide a common serialization/deserialization layer between RedisChannelLayer and PubSubRedisChannelLayer
2. Provide a way to customize how messages are serialized before being forwarded to the broker (aka: redis), which may address problems like Django Channels Memory Leak on every message or connection channels#2094

The serializer registry is somehow inspired by that of django.core.serializers package.
This also prepares a future in which msgpack would no longer be a mandatory dependency.

[carltongibson]

Oh, very exciting @sevdog — Thanks for thinking about this.

[sevdog]

I would like to know your opinion @carltongibson regarding my design choice to put encryption into the serialization process.

Also I noted that currently thare are no tests for that feature, I could add them to this PR if you consider that useful.

[carltongibson]

@sevdog Let me have a look/think. I've asked @bigfootjon and @acu192 to have a look too.

[bigfootjon]

I love the idea! I have a few questions and some requests but this is in great shape already!

[carltongibson]

This all looks quite reasonable.

I'm not quite seeing how it ties into the memory leak issue... 🤔 (Do we have an actual minimal reproduce for that anywhere?)

[carltongibson]

... know the precedence of operators...

Shouldn't have people need to stop and think here. +1 to the extra parentheses.

Although... 🤔 Could we check isinstance on a Protocol here? (Perhaps not but...)

[sevdog]

I belive the usage of Protocol could be added later, when a more extensive support for typing gets into the codebase.

[sevdog]

I'm not quite seeing how it ties into the memory leak issue... 🤔 (Do we have an actual minimal reproduce for that anywhere?)

My hypothesis in django/channels#2094 (comment) was that the cause of the issue was in msgpack (I suppose hidden somewhere in the C/Cython module, which is used by default when compiled, there should also be some memory problem with the Python implementation).
From this my idea to add a mechanism which allow simple customization of the serialization phase of the channel layers (which is also available in other libraries which use a broker, ie: celery/kombu).
Here is the code used in my tests for django/channels#2094: https://github.com/sevdog/django-channels-memory-leak (I just added a JS to open multiple websockets to have the same amount of requests in each tests and to do not have to reproduce it by hand, also I dockerized the application. There are variuos config which I left commented for readability).

This mechanism can achieve multiple goals:

• if there is a problem with a serializer you can switch to an other one (this can also be used for debugging purpose)
• you may choose between space-efficient (msgpack) or readable (json) format
• you may add a custom one without having to build a new channel-layer

[carltongibson]

OK thanks @sevdog — let me have a look at that. I have no objection to this proposal. 👍

[bigfootjon]

Great work! Sorry it took me a bit to get back to this and review.

Just a few comments left, mostly just cleanup at this point

[sevdog]

Changes pushed.

[bigfootjon]

LGTM! Thanks for working through all my comments @sevdog

@carltongibson, when you get a chance could you do a pass on this PR and merge if you agree?