Add ability to fetch only changes for incremental sync to device

[roeierez]

Ideally the client would pass the last sync timestamp with the outgoing payments query and settled invoices query and the service will reply only with the changes since that timestamp.

[cdecker]

We have been looking into pagination (ElementsProject/lightning#4687) and projection (ElementsProject/lightning#5679). I'm not quite sure how well these map to grpc, but it is certainly in the works.

One option we've been looking into is to have a top-level dict entry (along with id jsonrpc, method and params) called meta to pass those constraints in (offset, length limit, ...), and that could be mapped pretty easily to grpc metadata fields, thus allowing to pass them through.

For the shorter term we could provide server-side filtering (you tell us what your last sync-time was and we look for changes and stream them to the app), but I'd wait for the JSON-RPC discussion to settle, before getting started with mapping that to grpc metadata.

Would a change-stream with arbitrary time offset be interesting?

[roeierez]

All these in-progress items looks promising!
From the mobile perspective projection and flexibility in querying the remote data is not needed because we already use local sqlite that sync all the data from greenlight (we don't really have other choice if if don't want to loos offline support) so we can query locally. Even pagination is not really a requirement.
In fact we only need a way to sync efficiently so the answer is yes, gRPC stream with changes base on timestamp perfectly address this need . It will also reduce client boilerplate code that pull for changes and enable us to stream changes all the way (greenlight => breez sdk => app => UI)