Implement circuit breaking

[hadley]

Fixes #370

@atheriel I'd love your thoughts on this once you're back from break. Does this seem like a faithful translation of your httr PR?

• Figure out why that current implementation slows the tests down from ~13s to ~25s

[atheriel]

Minor comments but otherwise this looks pretty good to me.

[hadley]

Looks like slowdown is because req_perform tests take an extra 20s.

...

Seems to come from

test_that("don't retry curl errors by default", {
  req <- request("https://doesntexist") %>% req_retry(max_tries = 2)
  expect_error(req_perform(req), class = "httr2_failure")

  # But can opt-in to it
  req <- request("https://doesntexist") %>% req_retry(max_tries = 2, retry_on_failure = TRUE)
  cnd <- catch_cnd(req_perform(req), "httr2_retry")
  expect_equal(cnd$tries, 1)
})

....

Each of these requests now takes ~10s