Add memsector size computation logic.

Summary:
 - Remove --memsector-size flags and compute memsector size based
on temporary index.

Closes #24.

Test Plan: bin/mws-index -I data/zbl -o data/zbl_data

Reviewers: radu.hambasan

Reviewed By: radu.hambasan

Differential Revision: http://phab.code4fun.de/D60