RadosModel: use sharedptr_registry for snaps_in_use

There might be two concurrent rollback ops each of which
adds snap x to snaps_in_use.  Between when the first
completes and the second completes, snap x may be removed
since the first would have removed snap x from snaps_in_use.
Using sharedptr_registry here avoids this by ensuring that
the snap won't be removed from snaps_in_use until all refs
are gone.

This patch also adds size() to sharedptr_registry.

Fixes: #6719
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: David Zafman <david.zafman@inktank.com>