Reduce optimiseRandomTick new BlockPos instance create

[wling-art]

Reduce optimiseRandomTick new BlockPos instance creation

This patch optimizes the optimiseRandomTick method in ServerLevel by reusing a single MutableBlockPos instance per thread instead of creating a new immutable BlockPos object on every random tick.

• Introduces a ThreadLocal<BlockPos.MutableBlockPos> cache to avoid excessive object allocations.
• Updates the mutable position with .set(...) and converts it to an immutable BlockPos only when passing to randomTick.
• This reduces memory pressure and potential GC overhead during random block ticking.
• Ensures thread safety by using ThreadLocal to avoid concurrent modification issues.

Original PR: #332

[Dreeam-qwq]

Gooood