Spread out the signal reads so we're not updating all signals every X ticks

[narc0tiq]

Easy option: remember the tick when the monitor entity got placed and use placed_tick % sample_frequency_ticks as the key (spreads things semi-randomly, but probably sufficiently).

Trickier option: keep a bunch of buckets and divide round-robin into those buckets -- but they'll get uneven when removing monitor entities. Maybe load-emptiest-first? They still get uneven, but they get better when you add more.

Better yet: we're keeping a list of combis anyway -- just iterate that over multiple ticks, like the YARM ore reader (retaining the iterator between ticks). Possible difficulties:

• when fewer than signal_update_frequency monitors, we need to skip everything that's been updated more recently
• when more than signal_update_frequency monitors, we need to update more than one monitor at a time

(possible simplification opportunity: always iterate 4 combis every tick and don't allow the player to place more than 1200 combis)
(in fact, setting a maximum number of combis may be a good idea regardless, since both reading signals and storing them gets nontrivial as you add more and more)

[narc0tiq]

Not actually required for MVP.