Issue one time keys in upload order (#17903)

Currently, one-time-keys are issued in a somewhat random order. (In
practice, they are issued according to the lexicographical order of
their key IDs.) That can lead to a situation where a client gives up
hope of a given OTK ever being used, whilst it is still on the server.

Related: https://github.com/element-hq/element-meta/issues/2356