Enable subscribing to signals/events with an HTTP endpoint in statusd

[igor-sirotin]

Problem

Desktop and Mobile get signals/events from status-go by settings a c-binding callback here:

status-go/signal/signals.go

Line 24 in c9b777a

var mobileSignalHandler MobileSignalHandler

This is not possible to do with RPC functional tests, which use HTTP RPC API of statusd.
We should provide a way to subscribe to this events.

Implementation

There're 2 options:

1. WebSocket
   Implement a websocket that will simply publish all arising events.
2. Polling
   Implement an RPC endpoint PopEvents. All events are enqueued and can be popped with this endpoint.

The pros and cons are pretty obvious. WebSocket is a more robust solution, but might be more difficult to use in tests.

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Having such endpoint will greatly improve the abilities of the functional tests.

[igor-sirotin]

cc @antdanchenko