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.
Only lowercase and even-length tag values are stored as binary BLOBs.
Previously there was an error which search results from being returned
if the tag value was mixed-case and could be interpreted as hex.
A new database migration has been created to repair the `tag` table
for existing relays.
fixes: https://todo.sr.ht/~gheartsfield/nostr-rs-relay/37
