os_event_wait_time_low(): wait time calculation is messed up

Summary:
Feature: Core logic error

This is a fix for upstream bug#72520

In os_event_wait_time_low() the logic to calculate abs_time
for wait is broken. The bug has been present at least since
5.5. It gets acutely sensitized when sub-second wait intervals
are passed. It is particularly relevant to us because the
page_cleaner thread will mostly request sub-second wait
intervals. This can potentially lead to a near tight loop
behaviour of page_cleaner with much less sleep then what we'd
actually expect.

Test Plan: mtr

Reviewers: pivanof, steaphan, darnaut

Reviewed By: pivanof

CC: jtolmer, CalvinSun, WebScaleSQL

Differential Revision: https://reviews.facebook.net/D18459