vinyl: fix calculation of svindex->used and vy_range->used

* Fixes #1550 deadlock on ss_quota() during vinyl/stress.test.lua
* Fixes #1584 losing data after snapshot
* Fixes #1600 bad results from engine/recover_snapshot_wal.test.lua
* Fixes useless si_plannerpeek_branch() + si_branch() loops
  caused by false-positive si_plannerpeek_branch() results

This regression was introduced by 1edaeabb