librados: don't hold lock in dispatch()

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
librados: don't hold client log in certain operations

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
objecter: grab session write lock (fix)

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
objecter: take session lock when needed

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>