fix(core): make sure pausedMutations always execute in serial (#4902)

* fix: make sure pausedMutations always execute in serial

when resumePausedMutations is called multiple times, we are starting the second paused mutation even though the first one has not yet finished. This can happen when a focus event and an online event happen in short succession

* fix: always return the same Promise when resumePausedMutations is called