This skips the more complex pingcap/parser, and speeds up parsing speed
by 50%.

We have also refactored the KV delivery mechanism to use channels
directly, and revamped metrics:

- Make the metrics about engines into its own `engines` counter. The
  `tables` counter is exclusively about tables now.
- Removed `block_read_seconds`, `block_read_bytes`, `block_encode_seconds`
  since the concept of "block" no longer applies. Replaced by the
  equivalents named `row_***`.
- Removed `chunk_parser_read_row_seconds` for being overlapping with
  `row_read_seconds`.
- Changed `block_deliver_bytes` into a histogram vec, with kind=index
  or kind=data. Introduced `block_deliver_kv_pairs`.

Only kill Lightning if the whole chunk is imported exactly. The chunk
checkpoint may be recorded before a chunk is fully written, and this will
hit the failpoint more than 5 times.

This helps debugging some mysterious cancellation where the log is
inhibited.

Added IsReallyContextCanceledError() for code logic affected by error
type.