This is to improve responsiveness of link graph updates, whilst
avoiding being blocked waiting for updates to complete.
Previously, large numbers of cheap jobs resulted in poor responsiveness
as it took a long time for jobs to cycle round.
Add 'linkgraph' debug category.
Savegame format change for link graph jobs.
Change link graph scheduler to support more than one operation
per day, on _date_fract ticks other than SPAWN_JOIN_TICK.
Check if the job is still running one date fract tick before it
is due to join and if so pause the game until its done.
This avoids the main thread being blocked on a thread join.
Show if pause is due to link graph job in status bar, update network
messages.
This does not apply for network clients.
