worker: restrict size of emitted dataclips

[josephjclark]

Sending ~40mb json dataclips is blowing up lightning

We always planned to do something about this. It's just started being a problem.

• 10mb is probbaly the target size env var but and option controlled
• we're tallking about the size of the serialized JSON, the size on the wire
• streaming encoder?

What if the limit is blown?

• kill the run
• exit reason dataclip too large

Limit will have to apply to log messages too.

So it's a generic payload limit

logs don't need to be a kill, but run:complete does