vinyl: do not apply run_count_per_level to the last level

Currently, run_count_per_level index option is applied to each LSM tree
level. As a result, we may end up storing each key run_count_per_level
times at the last level alone, which would result in prohibitive space
amplification. To avoid that, let's ignore run_count_per_level for the
last level.

Note, we have to tweak quite a few vinyl tests, because they implicitly
relied on the fact that producing run_count_per_level dumps would never
trigger compaction.

Closes #3657