Check if the caller who made this IPC call has disconnected.
If called from outside the context of IPC, this does nothing.
< 0 if the caller is still connected. The time in ms
since the disconnection otherwise
If the client connected and specified a codec to use, then we will use this codec making
cellblocks to return. If the client did not specify a codec, we assume it does not support
cellblocks and will return all content protobuf'd (though it makes our serving slower).
We need to ask this question per call because a server could be hosting both clients that
support cellblocks while fielding requests from clients that do not.
True if the client supports cellblocks, else return all content in pb