Blender Git Loki

Attempt to address performances issues of task scheduler with lots of very small tasks.

This is partially based on Sergey's work from D2421, but pushing the things
a bit further. Basically:
- We keep a sheduler-counter of TODO tasks, which avoids us to do any
locking (even of the spinlock) when queue is empty, in workers.
- We spin/nanosleep a bit (less than a ms) when we cannot find a task,
before going into real condition-waiting sleep.
- We keep a counter of condition-sleeping threads, and only use
condition notifications in case we know some are waiting on it.

In other words, when no tasks are available, we spend a bit of time in a
rather high-activity but very cheap and totally lock-free loop, before
going into more expansive real condition-waiting sleep.

No noticeable speedup in complex production scene (barbershop one), here
master, D2421 and this code give roughly same performances (about 30%
slower in new than in old despgraph).

But with testfile from T50027 and new depsgraph, after initial bake,
with master I have ~14fps, with D2421 ~14.5fps, and with this code ~19.5fps.

Note that in theory, we could get completely rid of condition and stay
in the nanosleep loop, but this implies some rather high 'noise' (about
3% of CPU usage here with 8 cores), and going into condition-waiting
state after a few hundreds of micro-seconds does not give any measurable
slow down for me.

Also note that this code is only working on POSIX systems (so no Windows, not
sure how to do our nanosleeps on this OS :/ ).

Reviewers: sergey

Differential Revision: https://developer.blender.org/D2426