Adjust zebra-consensus Buffer sizes, and document Buffer hang risks

[teor2345]

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

zebra-consensus uses a lot of Buffered and Batched services, but we haven't really checked that their bounds are high enough.

The underlying issues are:

• If lots of tasks with data dependencies use a Buffer or Batch with a small bound, Zebra can hang.
• Buffers or Batches with small bounds can also limit concurrency, particularly if the underlying service does a lot of work in its call.

Describe the solution you'd like

• Review Buffer and Batch bounds across zebra-consensus

Document the Buffer and Batch constraints in:

• ChainVerifier
• BlockVerifier
• TransactionVerifier - 1 per transaction?
• CheckpointVerifier
• transaction::Verifier

See #1593 for details.

Describe alternatives you've considered

Do nothing: the code might hang if future changes break the poll_ready/call invariants.

Additional context

This issue was discovered during the review in #1593. It is a routine cleanup.

[teor2345]

This task has been revised based on the fixes to the underlying Buffer issue, see #1593 for details.

[teor2345]

The current settings seem fine.