One change to the contract, no engine behaviour, for dbhealth.

- A DatabaseTable sends schema_hash every report, a hash of its
  columns' names, types and nullability in column order, and
  schema_changes in the report where the hash moved and the two after:
  each column added, dropped, retyped or changed in nullability, with
  the type before and after, at most wire.MaxDatabaseSchemaChanges (60)
  a bundle. Its volume is what the system's counters say:
  rows_inserted_per_second, rows_updated_per_second and
  rows_deleted_per_second over the interval, beside the estimate.
- The widest database bundle measures 84 KiB; a receiver that accepts
  database bundles allows 128 KiB, what the load spec first asked for.
  The widest pipeline bundle is 9.0 KiB of its 10 KiB. (#448)