Improve view change support of deleted documents

[benoitc]

Right now the only way to replicate deleted document using the view changes is 👍

1. enable the options include_deleted
2. pass the properties matching the match function in the deleted document (or delete without emptying them using PUT and the _deleted property

The proposed change is to mark deleted keys in the views and keep them in the index until they are purged like the deleted documents in the main btree. The key associated to a deleted document ids will be marked as deleted and kept in the view index.