Rename `cache_` to `load_` as we're going to want real disk-based caching
Refactor methods onto `TaskList` that belong there
Got modified code from caldav date search that uses `REPORT` query to fetch all task data at once
Fetch `etag` with data, and support knowing the `etag` for a Task
Also fetch `etag` when doing a normal `load`
Use existing property methods rather than repeating code in `format`
Fixes from experimentation