Dump statistics while loading harvests.

Summary:
 - add [-s|--log-index-stats] flag to mwsd to allow logging basic
 statistics about the index while loading harvests. This allows us
 to track index efficiency as we load more documents.

Test Plan:
make test
bin/mwsd -I data/zbl -s stats.txt

Reviewers: radu.hambasan

Reviewed By: radu.hambasan

Differential Revision: http://phab.code4fun.de/D88