Replace some of context.sql.get_config() with context.get_config()

Pattern `context.sql.get_config(context, {foo})` is unnecessary
redundant in Rust: unlike C, Rust has associated functions (methods).

src/dc_imex.rs

@@ -362,7 +362,7 @@ fn set_self_key(
         error!(context, "File does not contain a private key.",);
     }
 
-    let self_addr = context.sql.get_config(context, "configured_addr");
+    let self_addr = context.get_config(Config::ConfiguredAddr);
 
     if self_addr.is_none() {
         error!(context, "Missing self addr");