Delete-from-metadata-query in metadata mixin

[hemidactylus]

Utility find_and_delete_entries method to delete items based on a metadata query.

Note: CQL does not support something like DELETE FROM table WHERE metadata['k']='v', i.e. SAI-based and no partition key supplied (and for good reasons).

But for some usages we want to expose such possibility in the metadata mixin of cassio.

Remark by Phil:

I would not be super-concerned about the Cassandra impact tbh…it will simply tombstone the record and compact out after gc_grace. I can’t imagine many vectors will be used in anti-patterns like message queues (rapid create/delete of a record would be a concern).