Full support for websocket reconnection/resubscription #3167

When using `.on<event>=fn` to attach listeners, only one listener can be set at the same time. Since multiple request managers can use the same provider, the EventTarget API has to be used to ensure all of them receive the events emitted from the provider. This is needed on both the `on` and `removeListener` functions.

The method `once` is required to allow the subscription logic to identify if the provider is able to reconnect/resubscribe and then attach to the following `connect` event the function to resubscribe.

When the subscription fails on start and when it fails after it was successfully established, the same logic needs to be executed: remove subscription, listen for the next `connect` event if available to actually subscribe again, emit the error and call the callback. Prior code did that only for established subscriptions so if a subscription was unable to be set right on start, no resubscription was ever tried. The logic was moved to a single method to avoid duplication of code. In addition reentry is avoided by checking and properly clearing the `_reconnectIntervalId` variable.

On subscribe, if there is an existing `id`, the subscription listeners are removed. In the case of a resubscription, the listeners have to be kept. Therefore, the `id` property -that will change anyway- must be cleared so the listeners are not removed. Then, after the subscription object resubscribes, the listeners set by the subscription user code remain untouched, making the resubscription transparent to the user code.

When the request manager removes a subscription due to an error, it tries to send an unsubscribe package, which can also fail if i.e. the network is down. In such a case, the function must not allow reentry. Removing the subscription first ensures it will not do so. In addition, if the subscription was already removed, the callback shall be called anyway.

When error events are emitted by the provider, all subscriptions shall receive the event and trigger the unsubscription/resubscription logic.

By wrapping the available WebSocket implementation (native WebSocket object or `websocket` package) with `websocket-reconnector`, the provider is given a WebSocket that will automatically reconnect on errors. A new option was added to the WebSocket provider to controll whether it should automatically reconnect or it should behave as usual.

In the case any websocket call takes too long to return and a timeout was set for the provider to timeout, the provider should try to restart the connection. This could happen, for instance, if the client loses connection with the server, the server closes the connection and later, the connectivity is up again but since the client did not receive the closing frame *and* the client does not attempt to send any package to the server, no error is observed. `websocket` implementation for Node.js has an option to send keep-alive frames and detect such scenarios, but the standard browser W3C WebSocket does not, so it is "vulnerable" to this kind of failure which will mostly affect web3 subscriptions.

If the provider silently recoonects and emits a new "connect" event, the subscriptions have to be set again over that new connection.

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Full support for websocket reconnection/resubscription #3167

Full support for websocket reconnection/resubscription #3167

Commits on Nov 6, 2019