Exec-next: make connections, scoping, and field extensions work

[rmosolgo]

Part of #5507

TODO:

• Make connections somehow work
• Update scoping test to use Batching
• Look for extensions tests, update them -- decide whether extensions should be supported at all?
  • Add new extension methods instead of .is_a? checks to distinguish? resolve_batch, after_resolve_batch?
• Zero extension overhead when not used (remove ExtendedState usage, no looping over empty extensions)
  • I didn't go to zero here, but the overhead is not measureable in my little benchmark.
• Document compatibility