[PATCH 0/7] vinyl: improve stats for throttling

[Vladimir Davydov]

I pushed the following approved patches plus the patch that sets the
default value of box.cfg.vinyl_write_threads to 4 to branch 1.10:

e78ebb77 vinyl: add global memory stats
16faada1 vinyl: fix accounting of secondary index cache statements
fe1e4694 vinyl: set box.cfg.vinyl_write_threads to 4 by default
7069eab5 vinyl: don't start scheduler fiber until local recovery is complete
0ff58856 vinyl: zap vy_worker_pool::idle_worker_count
3e76f7b9 vinyl: use separate thread pools for dump and compaction tasks
ba7abf6f vinyl: move worker allocation closer to task creation
49595189 vinyl: factor out worker pool from scheduler struct
661763ed vinyl: don't use mempool for allocating background tasks
04a735b2 vinyl: add helper to check whether dump is in progress

Here's what's left on branch dv/vy-stats-for-throttling:

a4e0694d vinyl: keep track of thread pool idle ratio
323161d9 vinyl: keep track of compaction queue length and debt
8468caa8 vinyl: update compact priority usual way on range split/coalesce
5d1b5db6 vinyl: fix force compaction logic
28d4f0f6 vinyl: add global disk stats