Database hash should include current datasette version

[simonw]

Right now deploying a new version of datasette doesn't invalidate existing URLs, so users may still see a cached copy of the old templates.

We can fix this by including the current datasette version in the input to the hash function (which currently just the database file contents).

[simonw]

A few extra thoughts:

• Some users may want to opt out of this. We could have --config version_in_hash:false
• should this affect the filename for the downloadable copy of the SQLite database? Maybe that should stay as just the hash of the contents, but that's a fair bit more complex
• What about users who stick with the same version of datasette but deploy changes to their custom templates - how can we help them cache bust? Maybe with --config cache_version:2

[simonw]

This is also a great excuse to finally write up some detailed documentation on Datasette's caching strategy