Data layer

[spirali]

This is experimental version of data layer (direct data transfers between tasks).

Main missing parts:

• Stabilized user API (the current one is just designed for testing)
• Some more advanced optimizations in scheduler
• Some optimizations in the data transfer itself

Because of never-ending conflicts I want to merge in the current form.
Even we would eventually remove the data layer (That is relatively easy, as now the code fully separates data dependencies and task dependencies)
then it is still to worth to merge it as this set of commits contains various generic refactorings, fixes problems in scheduler, fixes problem in some tests.

[Kobzol]

I went through all the commits, it sort of makes sense to me from a high-level of view, although I of course couldn't scan every little change in a 4k diff :) I think that I found one memory leak, the rest are just nits.

[Kobzol]

Welcome to the data age!