Improve Future cancellation story in `JsonRpcProvider` constructor.

[emizzle]

JsonRpcProvider is constructed with two futures:

type
  JsonRpcProvider* = ref object of Provider
    client: Future[RpcClient]
    subscriptions: Future[JsonRpcSubscriptions]

And in the constructor, another Future[void] is created for initialized:

proc new*(
  _: type JsonRpcProvider,
  url=defaultUrl,
  pollingInterval=defaultPollingInterval): JsonRpcProvider {.raises: [JsonRpcProviderError].} =

  var initialized: Future[void]
  var client: RpcClient
  var subscriptions: JsonRpcSubscriptions

  proc initialize() {.async: (raises: [JsonRpcProviderError, CancelledError]).} =
    convertError:
      case parseUri(url).scheme
      of "ws", "wss":
        let websocket = newRpcWebSocketClient(getHeaders = jsonHeaders)
        await websocket.connect(url)
        client = websocket
        subscriptions = JsonRpcSubscriptions.new(websocket)
      else:
        let http = newRpcHttpClient(getHeaders = jsonHeaders)
        await http.connect(url)
        client = http
        subscriptions = JsonRpcSubscriptions.new(http,
                                                pollingInterval = pollingInterval)
      subscriptions.start()

  proc awaitClient(): Future[RpcClient] {.
      async: (raises: [JsonRpcProviderError, CancelledError])
  .} =
    convertError:
      await initialized
      return client

  proc awaitSubscriptions(): Future[JsonRpcSubscriptions] {.
      async: (raises: [JsonRpcProviderError, CancelledError])
  .} =
    convertError:
      await initialized
      return subscriptions

  initialized = initialize()
  return JsonRpcProvider(client: awaitClient(), subscriptions: awaitSubscriptions())

If these futures are never completed/awaited, they could leak. This could happen anytime an exception is raised in the constructor, or if an exception is raised in awaitSubscriptions/awaitClient, or if JsonRpcProvider is successfully constructed but then the futures are not completed/awaited.

If exceptions are raised, the futures would need to be cancelled.

If the construction is successful, the futures would need to be cancelled on destruction.