This replaces the existing, unreleased lws_set_timer(wsi, secs) with
lws_set_timer_usecs(wsi, usecs).
wsi with a timer waiting are added to a linked-list sorted by the
timer trigger time.
1) poll() timeout (ie, poll wait) is trimmed to the nearest ms of the
first waiting timer if the default poll wait is longer than the
interval until the first waiting timer.
The linked-list of waiting timers is checked every entry and exit
from poll()... if no timers waiting or none reached their time
this costs almost nothing.
2) libuv: the earliest hrtimer is checked after every IO, again this
is costing nothing if the list head is NULL. If the case there
are hrtimers on the list, it costs a getimeofday (a VDSO in linux)
and more only if any of the timers have fired.
In addition on entry to libuv idle, if there are any waiting hrtimers
on the list, a libuv timer is used to force a wake in case we stay
idle (the libuv timer has ms resolution).
3) libev: not implemented
4) libevent: not implemented
Warnings are logged in the api is used on an event backend without
support. Patches welcome to add support similarly to libuv.
This adds a new api lws_set_timer(wsi, secs), which schedules
a callback LWS_CALLBACK_TIMER secs seconds into the future.
The timer can be continuously deferred by calling lws_set_timer()
again before it expires.
Calling lws_set_timer(wsi, -1) cancels any pending timer.
