Make Echo API validation asynchronous

[wrygiel]

Currently, when a user requests an Echo API validation on the Developers Page, the page is making a single request X to an Echo API validator endpoint of the DEV Registry Service. This endpoint then performs all the tests synchronously, while keeping the connection X open. Only after all tests are ready it responds to request X.

This works okay if the number of tests is limited. However, as the number of Echo API Validation tests grows, so does the risk of getting timeouts for request X. And the number grows quite fast, due to the nature of how they're generated (trying out all possible combinations of different security methods).

It might be a good idea to perform these tests asynchronously - request X would start the tests (and respond immediately), while the result would be fetched later on by making another separate request.