Support dataloader calls after batched lazy resolve

[rmosolgo]

Fixes #5399

Previously, the promises were all resolved inside a single fiber, so when a promise led to a Dataloader .load call, that fiber would pause until Dataloader ran. Then that fiber would resume again, make the next .load call, then pause again.

Now, each promise is handled in a separate fiber, so when it calls .load, Dataloader can continue running other fibers (and then it can accumulate other .load calls before running #fetch).