Proposal: get rid of the "collection" entries in Redis

[oli-h]

In Redis backend, vertx-rest-storage keeps track of the "path" to each resource by implicit keep "collection" entries up-to-date. As far as is see, those collections...

• are used for resource browsing/listing (see https://github.com/swisspush/vertx-rest-storage-editor)
• are used for the request feature "?expand="
• are not used when directly targeting a concrete resource
• need to be updated with every PUT/DELETE

Proposal: Use Redis "SCAN 0 MATCH " command to list 'entries in a directory' or complete subtrees.
This could also speedup deep expands, where, for now, vertx-rest-storage has to visit various collections (and finally resources) recursively.