sql: use semaphore to limit access to the connection pool

This ensures that if multiple connections are returned
to the pool at the same time, waiters get them in the order
they were placed in the queue.

src/sql.rs

@@ -338,7 +338,7 @@ impl Sql {
     pub(crate) async fn get_conn(&self) -> Result<PooledConnection> {
         let lock = self.pool.read().await;
         let pool = lock.as_ref().context("no SQL connection")?;
-        let conn = pool.get().await;
+        let conn = pool.get().await?;
 
         Ok(conn)
     }