vinyl: add last level size to statistics

In order to estimate space amplification of a vinyl database, we need to
know the size of data stored at the last LSM tree level. So this patch
adds such a counter both per index and globablly.

Per-index it is reported under disk.last_level, in rows, bytes, bytes
after compression, and pages, just like any other disk counter.

Globablly it is repoted in bytes only under disk.data_compacted. Note,
to be consistent with disk.data, it doesn't include the last level of
secondary indexes.