Support HTTP long polling

[imiric]

A recent support question in Slack brought up testing a SQS endpoint which uses HTTP long polling. k6/http currently only supports synchronous requests, and long polling is a popular feature used by other services as well, so we should look into supporting it.

From a brief discussion with @na--, this will likely require an event loop implementation per VU, which puts us on the road to support promises, async/await, etc., greatly expanding the testing capabilities.

Also see #746 and #779.

[na--]

I'd say that the event loop issue (#882) is the js-compat issue, this would just build on top of it. Though I'd also add evaluation needed, since we have to decide on the API eventually.