commit f8f364b47f2784f16b401f27658f1c16eaf348ec
Author: Jay Edgar <jkedgar@fb.com>
Date:   Tue Oct 17 15:19:31 2017 -0700

    Add a hashed, hierarchical, wheel timer implementation

    Summary:
    In order to implement idle timeouts on detached sessions we need something inside MySQL that is lightweight and can handle calling events in the future with very little cost for cancelling or resetting the event.  A hashed, hi

    By default the timers are grouped into 10ms buckets (the 'hashed' part), though the size of the buckets is configurable at the creation of the timer.  Each wheel (the 'wheel' part) maintains 256 buckets and cascades to the whe

    Reviewed By: djwatson

    Differential Revision: D6199806

    fbshipit-source-id: 5e1590f