Fix double-callback issue

[rgomezp]

Fixes #1015

A previous fix didn't work because of flawed logic. We were nulling the copy of the callback which was needed since the Callback argument was final.

This change utilizes a final callback array which works around the final object requirement since arrays can be final and still mutable.

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This change is 

[nan-li]

Nice that final was not needed.
Were you able to test? I can reproduce a callback firing twice, so I will test it out.

[jkasten2]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @nan-li)

[nan-li]

LGTM!
Noting that send outcome methods don't have callbacks checked but that should not be necessary because they shouldn't be invoked twice, unlike setting EUID.