The compact_heap, used by the scheduler to schedule range compaction,
contains all ranges except those that are currently being compacted.
Since the appropriate vy_index object is required to schedule a range
compaction, we have to store a pointer to the index a range belongs to
in vy_range->index. This makes it impossible to move vy_range struct and
its implementation to a separate source file.

To address this, let's rework the scheduler as follows:

 - Make compact_heap store indexes, not ranges. An index is prioritized
   by the greatest compact_priority among its ranges.

 - Add a heap of ranges to each index, prioritized by compact_priority.
   A range is removed from the heap while it's being compacted.

 - Do not remove indexes from dump_heap or compact_heap when a task is
   scheduled (otherwise we could only schedule one compaction per
   index). Instead just update the index position in the heaps.

Needed for #1906