vinyl: fix force compaction logic

This patch addresses a few problems index.compact() is suffering from,
namely:

 - When a range is split or coalesced, it should inherit the value of
   needs_compaction flag from the source ranges. Currently, the flag is
   cleared so that the resulting range may be not compacted.

 - If a range has no slices, we shouldn't set needs_compaction flag for
   it, because obviously it can't be compacted, but we do.

 - The needs_compaction flag should be cleared as soon as we schedule a
   range for compaction, not when all slices have been compacted into
   one, as we presently expect, because the latter may never happen
   under a write-intensive load.