Segregate history entries by "session"

[sagebind]

It would be a useful distinction to be able to page through just the command history for a single session, or for history of all sessions.

To do this we have to figure out what a "session" is:

• A random UUID generated whenever Riptide initializes. A meaningless value, but effective.
• PID + timestamp: this should be unique enough that it will differ for all logical sessions, and has the advantage of being more meaningful.

To avoid duplicate storage, we could create a new table that is basically session_history and have command_history entries hace a foreign key pointing to the originating session. We could move pid from the command entries to the session entry in this scenario assuming a session can't span multiple PIDs.

Are forks the same session, or a new one?