Fix poll_ready usage for the script::Verifier service

[teor2345]

This ticket was obsoleted by the fixes to the underlying issue in tower::Buffer, see #1593 for details.

Is your feature request related to a problem? Please describe.

The script::Verifier service uses poll_ready correctly, but in a risky way. (See #1593.)

The code works right now. But if the StateService is changed, the script::Verifier could fill up Buffers or Batches, potentially causing hangs.

Describe the solution you'd like

Make sure each poll_ready is followed by a call:

• Always return Poll::Ready(Ok(())) from script::Verifier::poll_ready
  • you can delete the current implementation - it's correct but risky
• Use ServiceExt::oneshot to call the StateService
• Document the poll_ready/call invariants to avoid future changes causing hangs

The documentation should look something like:

// Correctness:
//
// We avoid calling `poll_ready` on state service here, because that makes it easier to leak buffer slots.
// See #1593 for details.

See #1593 for a detailed background explanation.

Describe alternatives you've considered

Don't make any changes. Future changes might cause Zebra to hang.

Follow Up

Handle backpressure correctly (#1618)

Original Issues

Design: Make sure all poll_ready reservations are actually used in a call #1593
Example: Fix poll_ready usage for the Inbound service #1620

[teor2345]

@oxarbitrage you might want to work on these script::Verifier fixes after you've finished all your other high-priority tasks.

Or @yaahc you might want to work on this, if you finish the proof verifier before @oxarbitrage has finished his other work.

[teor2345]

This ticket was obsoleted by the fixes to the underlying issue in tower::Buffer, see #1593 for details.