Compactor improvements

[sandeepsukhani]

What this PR does / why we need it:
A couple of improvements:

1. During compaction, we create an empty file and merge the index from all the files into it. This wastes a lot of resources in a big cluster that has large index files. This PR changes it to use compacted file built from the last compaction as seed file and then copies index from all the other files into it. If there is no compacted file then it uses whatever files appear first in the list of files to compact. This should significantly reduce compaction time in a large cluster.
2. Allows configuring compactor to compact multiple tables at a time.

Checklist

• Tests updated

[cyriltovena]

LGTM