improvement: add NostrRepo trait, with sqlite implementation

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.

src/info.rs

@@ -37,7 +37,7 @@ impl From<config::Info> for RelayInfo {
             contact: i.contact,
             supported_nips: Some(vec![1, 2, 9, 11, 12, 15, 16, 20, 22]),
             software: Some("https://git.sr.ht/~gheartsfield/nostr-rs-relay".to_owned()),
-            version: CARGO_PKG_VERSION.map(|x| x.to_owned()),
+            version: CARGO_PKG_VERSION.map(std::borrow::ToOwned::to_owned),
         }
     }
 }