Make mr::Executor an adapter

[cone-forest]

Currently mr::Executor causes deadlock upon resizing.
This would be solved by either:

• improving existing thread pool solution.
• switching to off-the-shelf thread pool.
• throw this problem on the user.

The best way in my opinion is to:

1. abstract underlying mr::Executor implementation.
2. provide multiple implementations (existing, BS::thread_pool, inkooboo's thread pool)
3. provide CMake options to choose, what implementation should be used (use the best by default).