RadosClient: shutdown monclient after dropping lock

Otherwise, the monclient shutdown may deadlock waiting
on a context trying to take the RadosClient lock.

Fixes: #5897
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 0aacd10e2557c55021b5be72ddf39b9cea916be4)