`cancelAndWait` on `or` triggers deadlock

[etan-status]

this leads to deadlock:

import chronos

proc f(): Future[void] {.async.} =
  await sleepAsync(10.seconds) or sleepAsync(5.seconds)

proc g(): Future[void] {.async.} =
  let fx = f()
  await fx.cancelAndWait()

waitFor g()

echo "ok"

this works fine:

import chronos

proc f(): Future[void] {.async.} =
  discard await race(sleepAsync(10.seconds), sleepAsync(5.seconds))

proc g(): Future[void] {.async.} =
  let fx = f()
  await fx.cancelAndWait()

waitFor g()

echo "ok"