Implement Lease KeepAlive

[jeffa5]

This requires streams (similar to watches) to work.

[jeffa5]

Currently implemented as a unary call so it is possible to refresh the leases, though not ideal.

[heidihoward]

Is the plan to implement using unary or wait for bidirectional streaming support in CCF?

[jeffa5]

Due to the way bidirectional streaming (in particular the client streaming part) works I think this needs the same patch as for watches in #186. So a unary implementation doesn't seem to work.

[jeffa5]

Note, this works with http1 requests that aren't streaming (when running LSKV in http1 mode), but does not work with the etcd client (or gRPC streaming in general) on http2 as the client streaming logic isn't applied. I think (but haven't tested) that using a http2 client for the non-streaming style request would still work

[heidihoward]

Am I correct in saying that these are now supported (though a patch to CCF is still required)?

[jeffa5]

I believe it should be (or at least is in theory). I'll test things out once we get the rb maps remove in CCF main and I get a chance to rebase LSKV on that.