Framework- and sub-client version and timestamp setting

[bdpedigo]

Draft PR (mainly intended to spur discussion of what this should look like)

• Closes https://github.com/CAVEconnectome/CAVEclient/issues/191 (adds a property for the timestamp with a setter/getter) but now larger in scope than #193
• Adds documentation about the behavior with the default timestamp (previously it said this was always now, which wasn't necessarily true if someone made a chunkedgraph client with a timestamp)
• Makes it so that get_latest_roots respects the default timestamp - this is a change in logic but I think it makes sense