DeltaHandler duplicates storage of "ETag" values in Redis

[oli-h]

Gateleen's DeltaHandler explicit stores ETag values in Redis. See https://github.com/swisspush/gateleen/blob/v1.1.29/gateleen-delta/src/main/java/org/swisspush/gateleen/delta/DeltaHandler.java#L172

Those ETag values are stored as Redis-Keys with pattern delta:etags:... See https://github.com/swisspush/gateleen/blob/v1.1.29/gateleen-delta/src/main/java/org/swisspush/gateleen/delta/DeltaHandler.java#L44

Next to DeltaHandler, vertx-rest-storage module also stores ETag values for a resource - but it does it under a different key (typically ina Hash under Key rest-storage:resources:... and then as key "etag" within this Hash)

Note that there is a different behavior:

• vertx-rest-storage always generates an ETag value (for every PUT operation) - it generates a random UUID. Exception: if an ETag is provided as http request header if-none-match
• DeltaHandler never generates ETags by itself. It only uses a given ETags in form of http request header if-none-match

Propose to consolidate both ETag storage mechanisms