librbd: Add snapshot namespace to snapshot-name index in ImageCtx

librbd: Add snapshot namespace to snapshot-name index in ImageCtx

The essence of this huge change is to have two keys for indexing
snap ids in ImageCtx. It used to be a map of (snap_name -> snap_id)
now it's (snap_namespace, snap_name) -> snap_id. Therefore now snapshots
can have the same name if they are in different namespaces.
All the remaining changes are a consequece of this change of
ImageCtx.snap_ids field. The only exception is: we assume that you can't
rename snapshots from GroupSnapshotNamespaces. So rename operation
always assumes UserSnapshotNamespace.

Signed-off-by: Victor Denisov <denisovenator@gmail.com>