Synchronize yielding to fix jank

[toberge]

explicit synchronization is the way to go

• should the flickering yield sign problem
• won't avoid problems where clients try to yield/stop yielding but aren't in sync (would take more refactoring)
• should also fix some problems with the yield countdown not restarting maybe

[Fueredoriku]

Code looks good, great to have this fixed!! 👍
We should probably test this a bit thoroughly together before we deploy this (but the behaviour can't be much worse than it already is lol)