Optimize MultiPart and ParallelHatch performance

[krossgg]

What

This PR aims to optimize some parts of the Multiblock Lifecycle.

1. Whenever an IMultiPart needed to query the controllers it was attached to, it would iterate through all of its known controller positions and return a new List every time.
2. When the current parallel setting of a multiblock is needed, its parts list would be searched for the parallel hatch every time, which is especially noticeable when using many GCYM machines.

Implementation Details

1. As it is highly unlikely for an IMultiController to be moved from its current position without triggering a structure update and thus removing itself from the controller set, we can also keep track of a Set of IMultiControllers. I have used a ReferenceLinkedOpenHashSet from FastUtil, as we only really care about references and using a LinkedSet provides us with consistent iterations, as well as quick access to the first controller in the set.
   This Set does need to be populated manually on the client-side due to an inability to sync, so that is the reason why maintaining the controllerPositions set is useful.
2. It is very unlikely that the parallel hatch of a multiblock will change while it is formed, and thus can be cached. This has been implemented in the MultiblockControllerMachine class as a simple field. I have chosen to make the method return an Optional to better fit in with our current usages, but it can be switched to a Nullable if desired.

Additionally, some small tweaks have been added:

• RotorHolderPartMachines now cannot be shared and its methods have been updated to reflect that
• !getControllers.isEmpty() calls have been changed to isFormed()
• onUnload has been cleaned up a bit.

Outcome

Performance.

600s runs in fairly stressful situations
Shown is average MSPT spent executing the method
MultiPart example in RotorHolderPartMachine

Before	After

Parallel Hatch searching example in GTRecipeModifiers#hatchParallel
(ignore the Optional.map, it's the other bits that are relevant)

Before	After

[screret]

do the comment if you feel like it :)

[screret]

also add the @UnmodifiableView annotation here