Reject Eth subscriptions & filters through the gateway over HTTP

[Stebalien]

Currently, the lotus gateway forgets all eth subscriptions and event filters between HTTP calls. These APIs only work over the websocket transport.

This happens because:

1. We track filter and subscription IDs in the top-level HTTP request's context.
2. Even if we didn't do that, we couldn't guarantee that all HTTP requests hit the same API server (e.g., given a load balancer), so the websocket transport is the only reliable way to use these APIs anyways.

See

lotus/gateway/handler.go

Lines 89 to 96 in 9310cd6

	func (h RateLimiterHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	r = r.WithContext(context.WithValue(r.Context(), perConnLimiterKey, h.limiter))
	// also add a filter tracker to the context
	r = r.WithContext(context.WithValue(r.Context(), statefulCallTrackerKey, newStatefulCallTracker()))
	h.handler.ServeHTTP(w, r)
	}

Proposal: In the relevant Eth API methods (all those either taking or returning an EthFilterID and/or an EthSubscriptionID), check if the request is being made via a websocket and, if not, reject it with an error explaining that the API is only supported over the websocket transport.

Motivation: Users aren't confused because this just silently refuses to work. Instead, users will get a nice error message telling them to use a websocket if they need to use this API.