This is an experimental change to see if we can reduce memory usage
with large SQLite databases. If successful, we'll do this again and
further reduce the database mmap size.
This will cause greater use of the page cache, but that is more easily
reclaimed by the kernel, and should reduce memory pressure, as well as
making it clearer how much memory the application is actually using
for connections, subscriptions, etc.
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.
