Allow clippy warning for UnsafeCell in Arc

[timvisee]

Our master currently fails to pass CI because clippy introduced a new warning.

It suggests not to use Arc<UnsafeCell<_>> because UnsafeCell implements !Sync, implying that Rc should be used rather than Arc.

However, the wrapping type is Send, Sync, Clone thus using an Rc is not an option.

Using SyncUnsafeCell would resolve the warning, but sadly this is not stabilized yet.

I carefully inspected the current implementation for soundness, and all seems to be good. We never use overlapping mutable slices. Though, I do think that we should restructure these core types to make better use of the type system and make misusing these types a lot harder.

Note that this also fixes use of a deprecated chrono function as it also prevents CI from succeeding.

This is currently blocking:

• set create file option to false on opening a segment file just created #56
• Mmap flush does not require mutable reference, use corret mut ref self #58
• Remove getter/setter from Segment #59
• Update memmap2 requirement from 0.7.0 to 0.8.0 #60