Make Entities::needs_flush take &self instead of &mut self

[2ne1ugly]

Objective

• Fixes clippy::debug-assert-with-mut-call in Entities::verify_flushed() #6474

Solution

• Use load() instead of get_mut() at the atomic i64
• Also change verify_flushed as &self

[DJMcNab]

This only taking &self makes these methods not quite useless, but much more error prone. That's because using the result of this without being in a context where you have exclusive access to the entities means that the window where the results are guaranteed to be valid is empty, since another thread could do another operation inbetween the check and use of this property to make it invalid.

verify_flushed provides an easy way to see this, since it purports to confirm that the Entities is flushed but doesn't actually ensure this for any length of time.

In this case, we're using &mut T not as 'can be mutated', but merely as validation of exclusivity; clippy doesn't understand the difference between those (as the language doesn't make the distinction). Clippy is wrong here. This is the first case of rust-lang/rust-clippy#5112

Additionally, of course, this adds more unneeded atomic loads into a place, although that's a secondary concern.

[2ne1ugly]

I guess the another way to avoid the clippy warning is to just do it the regular way:
call function outside of debug and pass as variable

then we could "silence" the warning