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/utils.rs

@@ -2,7 +2,7 @@
 use std::time::SystemTime;
 
 /// Seconds since 1970.
-pub fn unix_time() -> u64 {
+#[must_use] pub fn unix_time() -> u64 {
     SystemTime::now()
         .duration_since(SystemTime::UNIX_EPOCH)
         .map(|x| x.as_secs())
@@ -10,12 +10,12 @@ pub fn unix_time() -> u64 {
 }
 
 /// Check if a string contains only hex characters.
-pub fn is_hex(s: &str) -> bool {
+#[must_use] pub fn is_hex(s: &str) -> bool {
     s.chars().all(|x| char::is_ascii_hexdigit(&x))
 }
 
 /// Check if a string contains only lower-case hex chars.
-pub fn is_lower_hex(s: &str) -> bool {
+#[must_use] pub fn is_lower_hex(s: &str) -> bool {
     s.chars().all(|x| {
         (char::is_ascii_lowercase(&x) || char::is_ascii_digit(&x)) && char::is_ascii_hexdigit(&x)
     })