QueryingSubscriber (to receive past publications)

[JEnoch]

Some use cases require a Subscriber to also receive the data that have been published before its creation.

Using the zenoh primitives, this can be done with a "QueryingSubscriber" that would:

• do a query at start-up to retrieve the historical data
• declare the subscription at the same time to receive live publications, but not delivering those to the user for now
• merge the query results and the live data into a same queue, filtering duplicates and ordering by timestamps
• deliver the content of this queue to the user
• finally, go on delivering live publications to the user

The queryable storing the past publications and replying to the query could be implemented in different ways:

• a zenoh storage deployed in a zenoh router
• or a "publication cache" colocalized with the publisher (to support peer-to-peer use cases without any router)

[JEnoch]

Implemented in zenoh-net API.
An implementation in zenoh API will come later.