Opportunistic low-traffic vacuum

[jberkus]

Depends On: #6, #5

Rewrite flexible freeze so that instead of using prescheduled low traffic periods, it can monitor for and detect prolonged low traffic periods and kick in then. The user would instead schedule known high traffic periods as blocked out periods.

This would be pretty tough to carry out, and might not end up being a good idea. If we get to this level, it's probably better to look at a PostgreSQL patch.