Make Multilevel adapter apply deletes in reverse order

[martosaur]

This PR changes the order of delete operations in the Multilevel adapter. Consider the following scenario:

1. Multilevel cache has a stale value
2. Caller A issues a delete request for the key.
3. Multilevel deletes the key from L1
4. At the same time Client B tries to get the key from cache. It finds it in L2 and replicates value to L1.
5. Multilevel delete the key from L2.

So we end up with stale key in L1 after delete.

In order to fix this it would make sense to perform deletes in reverse order, i.e. from L2 to L1. I figured out there are 3 callbacks affected: delete, take and delete_all.

Please let me know what you think!

[cabol]

Yeah, that makes sense, the PR LGTM!

[coveralls]

Coverage remained the same at 100.0% when pulling 6ec27ea on martosaur:reverse_multilevel_deletes into d0ff450 on cabol:master.