[Syncevolution-issues] [Bug 6175] Implement client side Server Alerted Sync handling

> I don't know what the effect of not using the suggested session ID is. I marked > it as a TODO because I saw that the SAN message contains it. Some servers might > depend on it. Checking the spec, I don't think session ID was meant to be used like this. The ID is actually used to help the client to identify the same SAN requests from a server (As the server might sent multiple SAN continuously) and thus only need to respond to one of the requests. Again this is not true for SyncEvolution Server at this moment, on the server side we might first ensure the generated session ID need to be unique. On the clietn side, I think we can implement like this: If the SAN request with the same session ID was received within a reasonable time period (say 1 hour), we should ignore this packet.