datacache: make m_MDLDict threadsafe (other things still are not!)

[RaphaelIT7]

Depends on #159

[dimhotepus]

I will close these thread-safety PRs for now.

The problem is they make entities more threadsafe, but not fully threadsafe.
I think we should follow another approach - these things should be const + member functions should be const.

If all is const, it means entities are immutable and can be used anywhere without synchronization overhead

If logically or by ThreadSanitizer we find a real bug, such PRs will be accepted.

[RaphaelIT7]

The problem is they make entities more threadsafe, but not fully threadsafe.
The focus for these changes mainly was on loading- when joining I've tested them.

If you'd ever want to test for these parallel issues
I locally had this commit applied to my rmod branch, using which I've then made all these changes.
The specific change: RaphaelIT7@c840dea#diff-583412cae94d9826fc2266aaf0c6d6f40beeee30f61d0468b9537773f4e900e3
(It's quite good at showing threading issues xd)

I'll definitely look into seeing which things to make const & such and see how far I get with your suggestion :D