spanner: allow moving sessions between clients

[danielnorberg]

We spawn subprocesses that need to perform latency critical spanner queries immediately on startup. One of the impediments to this setup is that creating a spanner session can be a slow operation.

Would it be possible to allow moving sessions from one client to another? Then the parent process could prepare sessions ahead of time and pass them to the subprocess on startup. I.e., take Session protos holding the session name from the parent process spanner client then serialize and pass them to the subprocess which would then deserialize and pass them in when creating a new spanner client.