Fix contact name update rules

The following rules apply now:
1. "name" column is only updated manually and never over the network
2. "authname" column is only updated over the network and never manually
3. Displayname is a "name" if it is non-empty, otherwise it is "authname".

This fixes a known (pytest.xfail) problem of "name" being changed over
the network when user has set it to non-empty string manually.

This also fixes the problem when "name" and "authname"
became unsynchronized accidentally, when they were equal and then
Origin::IncomingUnknownTo update arrived, setting "name" but not
"authname". Rust regression test is added for this case.

src/sql.rs

@@ -1491,6 +1491,15 @@ CREATE INDEX devmsglabels_index1 ON devmsglabels (label);
             }
             sql.set_raw_config_int(context, "dbversion", 73).await?;
         }
+        if dbversion < 74 {
+            info!(context, "[migration] v74");
+            sql.execute(
+                "UPDATE contacts SET name='' WHERE name=authname",
+                paramsv![],
+            )
+            .await?;
+            sql.set_raw_config_int(context, "dbversion", 74).await?;
+        }
 
         // (2) updates that require high-level objects
         // (the structure is complete now and all objects are usable)