This updates the DB schema to remove the distinction between hex and
non-hex tag values, for simplicity. The space savings did not seem to
be worth the extra complexity.
The SQLite tags table is denormalized to duplicate kind/created_at to
improve the ability of tag indexes to filter data.
This is inspired by the work of
v0l (https://github.com/v0l/nostr-rs-relay/).
A new trait abstracts the storage layer with an async API. Rusqlite
is still used with worker threads, but this allows for Postgresql or
other backends to be used.
There may be bugs, this has not been rigorously tested.
