Remove redundant operations in BaseClient

[garvenlee]

1. Make BaseClient._handlers a plain dict
2. Simplify read operations in the underlaying connection

About point 1:
Note that each client corresponds to a connection, but there may be multiple publishers or subscribers sharing this client with multiple PublishConfirm or Deliver callbacks registered into. When scheduling frame callbacks in _listener task, rstream needs a for loop, but only one callback actually process this frame, so the for loop is unnecessary. To solve this, Client needs to maintain extra info about stream topo.

# BaseClient
async def _listener(self):
    …
    handler = self._handlers.get(frame.__class__)
    if handler is not None:
        await handler(frame, self)  # add new arg `Client`

# Producer
async def _get_or_create_publisher(self, stream, publisher_name):
    …
    client = await self._get_or_create_client(stream)
    publisher_id = client.next_publisher_id
    client.publishers_info[publisher_id] = stream

async def _on_publish_confirm(self, frame, client):
    stream = client.publishers_info[frame.publisher_id]
    publisher = self._publishers[stream]
    …

# Consumer
async def _create_subscriber(self, stream):
    …
    client = await self._get_or_create_client(stream)
    subscription_id = client.next_subscription_id
    client.subscriptions_info[subscription_id] = reference

async def _on_deliver(self, frame, client):
    reference = client.subscriptions_info[frame.subscription_id]
    subscriber = self._subscribers[reference]
    …

About point 2:
Connection is based on Asyncio Stream API. Currently, Connection holds a buffer, and StreamReader also holds a buffer, in this way, data is copied twice in Connection._read. Maybe rewrite like this:

async def _read(self, n: int):
    try:
        return await self._reader.readexactly(n)
    except IncompleteReadError:
        raise ConnectionClosed

[DanielePalaia]

I'll close this issue as at the moment we are not planning this implementation. Anyway any contribution or PR submission is welcomed!