Can we use multifetch in req_fetch()? about httr2 HOT 1 OPEN

Currently four limitations documented in multi_req_perform():

1. Will not retrieve a new OAuth token if it expires part way through the requests.
2. Does not perform throttling with req_throttle().
3. Does not attempt retries as described by req_retry().
4. Consults the cache set by req_cache() before/after all requests.

But also:

5. Doesn't update last request/response
6. Doesn't support mocking (which will be done in #30)

So, need to add:

• Some way to perform a request in the future — need a separate queue that also tracks time "after" which request can be next added. If kept in order, can Sys.sleep() until the time stored in the first element. Generally expect that relatively few requests will end up in this queue so can be implemented naively. Used for throttling and retries.

• Some way to track number of attempts for each request.

• When processing error result, need some way to add back to pool or queue. This is connected to #8.

• Update the last request/response

from httr2.