WebsocketProviderV2 recv() lock + other multi-tasking support

[fselmo]

What was wrong?

Some use cases for WebsocketProviderV2 don't allow asyncio.gather(tasks...), for example, as it will halt when two coroutines try to call recv() on the websocket connection at once. This PR attempts to ameliorate these scenarios.

• Add an asyncio.Lock around the websocket recv() method.

• Change listen_to_websocket() so that it only listens to subscription (or one-to-many request / response) messages. This is because every make_request() expects a 1-to-1 response-to-request and expects it to be returned on the same line e.g. variable_assignment = await w3.eth.block_number. If we are persistently listening to the socket, we clearly expect many messages. So, we only listen to messages from one-to-many calls, triggered by a ws.send().

• Add an API for retrieving active subscriptions: w3.ws.subscriptions (WebsocketConnection.subscriptions)

• Because of the separation of 1-to-1 request / response and one-to-many send() to recv() messages, separate the RequestProcessor caches into two types of "caches". The subscription cache is really a deque that always uses popleft(), so make it one. The response cache is looking for a particular cache key so this still makes sense to keep as an OrderedDict.

• Defines an __await__() on the _PersistentConnectionWeb3 class so the AsyncWeb3.persistent_websocket() instantiation can be awaited
  (i.e. w3 = await AsyncWeb3.persistent_websocket(WebsocketProviderV2(...))). New integration test suite added with this pattern of instantiation.

Todo:

• Add entry to the release notes
• Add tests for new additions

Cute Animal Picture

snake socks

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[reedsa]

Nothing major, looks good overall!

[reedsa]

The comments that follow kind of look like code that was commented out. Recommend creating comment blocks that call these examples out more explicitly.

[fselmo]

I really only meant these to be for organization / reference. The websockets library does a similar thing with their connect() method here. I feel like we could let the documentation do the explaining. Thoughts?

[pacrob]

I like these examples of usage. Would a label, something like # Example usage: or # Access this method with pattern: make it more clear for users?

[fselmo]

I'd like to leave that up to the documentation. This is more so we separate the logic for each of those design patterns into code blocks and so contributors know where to look for it.

[pacrob]

Nothing came up in messing with it. Once existing notes are handled, lgtm!