implement caching for getNetworks function

[DaRacci]

Instead of constructing a new immutable collection, that is wrapping a new hash set copy multiple times every tick the immutable set gets cached and only recreated upon changes that dirty the master set.

This improves the total time spent each tick and also a major memory improvement.

Before changes

Time spent per tick

Memory Allocation per second & Total over 5 minutes

Post changes

Time spent per tick

Memory Allocation per second & Total over 5 minutes

[CLAassistant]

All committers have signed the CLA.

[coveralls]

coverage: 45.159% (+0.07%) from 45.087%
when pulling 7e5adf9 on DaRacci:performance-fix
into cd718d8 on CyclopsMC:master-1.21-lts.

[rubensworks]

Nice, thanks @DaRacci. Very impactful for servers with many networks.