librados: implement/document tmap_{get,put}

librados: implement/document tmap_{get,put}

These aren't strictly necessary now (you can just read the raw object or
do a writefull and get the same thing) but this way we document the format
and can change the backend to be smarter in the future without changing up
the interface.

Signed-off-by: Sage Weil <sage@newdream.net>