Ivar.fill crash fix: Remove outstanding_requests ivars during libp2p_helper shutdown/restart

[mrmr1993]

This PR removes the Ivar.ts from outstanding_requests in Mina_net2 when a shutdown is detected.

This avoids a double-fill when

• we perform a scheduled restart of libp2p_helper (or it crashes and we restart it) in Gossip_net.Libp2p.create
• there is some message read from stdout of libp2p_helper, and the scheduler yields before handling it
• the termination handler in Mina_net2 fills all unfilled Ivar.ts with an error state
• the scheduler resumes the message handling, which attempts to fill the now-filled Ivar.t.

A backtrace indicating this logic (or, at least, the final step) was posted on discord in the development channel.

Checklist:

• Document code purpose, how to use it
  • Mention expected invariants, implicit constraints
• Tests were added for the new behavior
  • Document test purpose, significance of failures
  • Test names should reflect their purpose
• All tests pass (CI will check this if you didn't)
• Serialized types are in stable-versioned modules
• Does this close issues? List them: