vinyl: fix background upsert squash

This patch is a quick and dirty fix for the following commits:

  3bda8943 vinyl: use one fiber for optimizing upserts
  e132110b vinyl: avoid scheduling upsert optimizer after each upsert
  3940cba2 vinyl: do not check index/range version when optimizing upserts

What it does:
 - Do not reset vy_stmt->n_upserts counter after scheduling squashing
   (when n_upserts is set to VY_UPSERT_INF).
 - In vy_squash_process(): apply statements that are inserted while we
   are reading an on-disk run. This is a dirty hack. Instead, we should
   fix the read iterator to handle this case.
 - Log errors if vy_squash_process() fails.