fix(repo/config) validate against struct before writing to disk

When setting config keys, the program doesn't know whether the
key-to-be-modified exists on the Config struct. (Perhaps, with
reflection, it is possible to find the field). To allow callers to write
non-existent keys, the program would...

Before:
1) converts the in-memory *Config to a map
2) sets the key on the map, and
3) writes this map to disk.
4) Then, it converts this map back into an in-memory struct.

This commit swaps 3 and 4 so the map can be validated against the struct
before being written to disk. This prevents the bug identified in #740.

repo/fsrepo/component/config.go

@@ -93,15 +93,15 @@ func (c *ConfigComponent) SetConfigKey(key string, value interface{}) error {
 	if err := common.MapSetKV(mapconf, key, value); err != nil {
 		return err
 	}
-	if err := serialize.WriteConfigFile(filename, mapconf); err != nil {
-		return err
-	}
 	// in order to get the updated values, read updated config from the
 	// file-system.
 	conf, err := config.FromMap(mapconf)
 	if err != nil {
 		return err
 	}
+	if err := serialize.WriteConfigFile(filename, mapconf); err != nil {
+		return err
+	}
 	return c.setConfigUnsynced(conf) // TODO roll this into this method
 }