Embed: Prevent redundant verification requests from the widget component

[nutrina]

There are cases where multiple requests are triggered from the UI widget component to the embed service.

Given that this processing is is intensive, we should prevent executing the handler for identical requests if the previous request is still running.

Potential solution: should store a flag in redis when a request handler starts for an address and invalidate that when the request ends (regardless if successfully or with error). In between we should return 429 for every request coming in for that same address). Multiple requests could be triggered by errors in the UI integration of the component.