vinyl: implement single memory level

 - In-memory trees are now created per index, not per range as before.
 - Dump is scheduled per index and writes the whole in-memory tree to a
   single run file. Upon completion it creates a slice for each range of
   the index.
 - Compaction is scheduled per range as before, but now it doesn't
   include in-memory trees, only on-disk runs (via slices). Compaction
   and dump of the same index can happen simultaneously.
 - Range split, just like coalescing, is done immediately by creating
   new slices and doesn't require long-term operations involving disk
   writes.