Add flag to clear existing data.

Summary:
 - add [-f|--delete-old-data] flag to bin/mwsd binary which
 clears existing data
 - this is useful when re-using a data output directory

closes #15.

Test Plan:
./bin/mwsd -I data/zbl --leveldb
./bin/mwsd -I data/zbl --leveldb
./bin/mwsd -I data/zbl --leveldb --delete-old-data

Reviewers: radu.hambasan

Differential Revision: http://phab.code4fun.de/D56