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/chat.rs

@@ -907,7 +907,8 @@ impl ChatId {
 
     async fn parent_query<T, F>(self, context: &Context, fields: &str, f: F) -> Result<Option<T>>
     where
-        F: FnOnce(&rusqlite::Row) -> rusqlite::Result<T>,
+        F: Send + FnOnce(&rusqlite::Row) -> rusqlite::Result<T>,
+        T: Send + 'static,
     {
         let sql = &context.sql;
         let query = format!(