Improve sync and import performance

[cloverich]

Now that Chronicles stores note contents as markdown files, and indexes over them, the database is a cache that needs kept in sync; it has a sync routine for this (it is very similar to importer, so everything here applies to both). Its mostly naive and non-performant because its not important enough to do right now, but here's some ideas:

1. Make the sync / import routine concurrent. Should be pretty straight forward. Likely would do w/ workers to make it easy to cap resource usage
2. Some kind of content hashing to quickly determine if a file needs re-indexed or not
3. (done) don't descend into directories needlessly
4. allow configuring which directories and prefixes to ignore (or perhaps, which to sync)
5. Do not delete indexed data when run starts (update during and after)
6. Run in separate process