* change `poll_ready()` and `call()` of `ChainVerifier`
* add bound, move max_checkpoint_height
* add buffers to the checkpoint and block verifiers
And rename the chain verifier fields so `block` means `Arc<Block>`, and
`block_verifier` means `Buffer<BlockVerifier, ...>`.
* Fix the error types
* Use `ServiceExt::oneshot` in `ChainVerifier::call`
And:
* make the code look like the `main` branch as much as possible
* document the `poll_ready`/`call` invariant
* Use `ServiceExt::oneshot` in `chain::init`
Co-authored-by: teor <teor@riseup.net>
0723ac5be1