Mark variables as volatile for safe concurrent access

[pjfanning]

If this change is ok I can port it to jackson-jaxrs-providers.

Shared variables that have mutable values are best marked as volatile. The _defaultAnnotationsToUse variable is only set in the constructors so it feels best to mark it as final to highlight that the value doesn't change and does not need to be volatile.

[cowtowncoder]

Sounds reasonable; access is messy and I don't really remember original reasoning. But agreed that it is safer to mark as volatile.

The only open thing is that one comment, suggesting PR was intended for 2.17 branch (or needs to updated). Either way is with me, just LMK.

[pjfanning]

This is a follow up to #26 and with that PR brining in optimistic locking behaviour, it becomes more important to have these changes.

[cowtowncoder]

@pjfanning I understand but my question was specifically if:

1. This should be against 2.18 branch (if so, comment should be updated to reflect this)
2. This should be against 2.17 branch (comment ok, PR needs to be rebased or recreated)

so I'm fine merging it but it seems to be in inconsistent state. I could solve it either way but not sure which of above is the intent.

Hope this makes more sense.

[pjfanning]

I'd prefer this to be in 2.17.1. If we choose to delay this to 2.18 then I would prefer to revert #26 and delay it to 2.18 too.

[cowtowncoder]

Ok. I suppose I should then merge this & cherry-pick, least amount of work.

[cowtowncoder]

Done.