added some consts, some cleanup

Signed-off-by: Michal Jarzabek <stiopa@gmail.com>
@dachary, @stiopaa1 I don't think this should be a const bufferlist. Granted the actual bufferlist we're passing in is not modified, but we copy it (and therefore the buffers it refers to) into a map which is used by a whole different thread to store response data. Declaring it const is at least misleading.

Am I missing something?