Allow "batching" of advance requests to reduce message count

[narinearraul]

mkistler commented on Mar 16, 2016
The client must send regular advance messages to the server to check for missing data, and advance is specific to ACL, so for a user that has read access to n ACLs, this means n regular messages from client to server and back every advance interval (currently 5 seconds). We should find some way to batch some or all advance requests in one message to reduce message traffic between client and server.