Merge lazy resolution into Dataloader

[rmosolgo]

Extracted from #5389

Unify resolution of Lazy objects (Promises) with Dataloader.

TODO:

• Track down remaining incompatibilities
• Add Resolve back to the project with deprecation warnings
• Remove steps_to_rerun_after_lazy if it's unused

cc @gmac perhaps relevant to your interests, this change would make dataloader.run do everything

[gmac]

That’s interesting… so I could just go through and queue a whole bunch of evaluate_selections across fields and scopes, then just run dataloader once at the end of everything and then go through collecting results? Sound like a nice simplification!

[rmosolgo]

Performance was pretty bad with FlatDataloader (extracted from #5389) because it was creating Procs from passed-in blocks. Instead, I moved Lazy resolution into the existing NullDataloader and I got nearly identical performance:

-                          14.140 (± 0.0%) i/s   (70.72 ms/i) -    142.000 in  10.047317s
+                          14.245 (± 0.0%) i/s   (70.20 ms/i) -    143.000 in  10.040795s

  Total allocated: 9424424 bytes (75415 objects)

(Same memory footprint)