This mitigates a side effect of the global current_time and monotonic_time being updated on every server call's timeout, where the end time of any unrelated server call is moved into the future (depending on the frequency of server calls).
By using a more granular timeout, the overall frequency of server calls doesn't have as great of an effect on each individual timeout, as we don't have to wait for an entire millisecond (which was due to the ceiling operation in get_next_timeout).
This [issue](https://bugs.winehq.org/show_bug.cgi?id=57849) gives a few examples of this causing significant degradation in the behavior of some games, which use alertable (server) waits for their FPS limiter.
-- v5: server: Use epoll_pwait2 if supported. server: Convert to and use a timespec directly for timeouts if supported.