fix: keep synchronized mutation executed in order

[huozhi]

Fixes #566

[ranisalt]

This has flipped the issue for me.

Before the patch, after every request is finished and a call to mutate is issued, it would overwrite everything that was mutated before it (the parameter to the callback passed to mutate is the original value fetched, not the latest state), so the last call to mutate would contain the original state mutated by the last call to mutate only.

After this patch, the intermediate state is wrong, as every request finished suffers from the same issue of only mutating from the original state, but the final result is the correct, up to date state. See the reproduction:

[huozhi]

hi @ranisalt , sorry for my limited understanding about your problem here. would you mind giving more explanation of the incorrect behavior of this PR you mentioned? like some code samples will be super helpful, thanks

[huozhi]

close this in favor of #735