perf: Budget less memory for Replay Checkpoints (#28052)

[tom-leys]

• Half the checkpoint RAM budget overall (1GB instead of 2GB)
• Simple change of the CVars and some stats so checkpoint creation triggers less frequently
• Based on a Lizard replay, checkpoints move from on average every 70 ticks to every 350.
• Added a minimum number of ticks between any two checkpoints (default 60 ticks)
• More likely to use a checkpoint when panning through the timeline (previously you had to jump quite far for it to look for a checkpoint)

Relates to https://github.com/space-wizards/space-station-14/issues/28052

Before (6.33GB before final load):

After (5.35GB total):

Naturally this will reduce seek performance a bit.