Cleanups for production

* Update comments and docs.
* Make cron directory so build succeeds
* Change to using flock(2) with a separate lockfile to avoid issues with the lock going along with the rename
First cut at adding db_iter support for sketches
Add sketch support to db_iter client bindings
Rev version to 0.7.1
Merge branch 'feature/sketches' of github.com:stevedh/readingdb into feature/sketches
Add support for a -4 flag, which enables support for IPv4-only environments