sql: replace .get_conn() interface with .call()

.call() interface is safer because it ensures
that blocking operations on SQL connection
are called within tokio::task::block_in_place().

Previously some code called blocking operations
in async context, e.g. add_parts() in receive_imf module.

The underlying implementation of .call()
can later be replaced with an implementation
that does not require block_in_place(),
e.g. a worker pool,
without changing the code using the .call() interface.

src/peerstate.rs

@@ -186,7 +186,7 @@ impl Peerstate {
     async fn from_stmt(
         context: &Context,
         query: &str,
-        params: impl rusqlite::Params,
+        params: impl rusqlite::Params + Send,
     ) -> Result<Option<Peerstate>> {
         let peerstate = context
             .sql