Should store intervals in DB, rather than ticks

[msteffen]

Currently, after each write, the watch daemon appends a row to its database. Instead, I think it would use less space and possibly speed up reads if it read the most recent interval/tick and appended to it (or started a new interval).

It would probably need to store initial writes as 0-length intervals, and remove the most recent interval inside of each write if it's 0-length and the current write doesn't extend it