the parsing offset for new instance fields was wrong, leading to some
instances of messages being missing, for example, MAVExplorer not
setting GPS[1] messages in tab completion

Signing is expensive both in code size and stack usage

Invalid argument gets spewed otherwise

Closes https://github.com/ArduPilot/ardupilot/issues/20251

use a dictionary of sets to record which instances of a message have
been seen before. This avoids us having to parse all instance based
messages fully during the load.

as we have added more instance messages the time to load the log has
become progressively longer. With this change it now becomes short
again

on a recent 160MByte quadplane log the time to load the log with this
change drops from 39.9s to 6.5s

and add test suite

Bumps [ajv](https://github.com/ajv-validator/ajv) from 6.10.2 to 6.12.6.
- [Release notes](https://github.com/ajv-validator/ajv/releases)
- [Commits](https://github.com/ajv-validator/ajv/compare/v6.10.2...v6.12.6

)

---
updated-dependencies:
- dependency-name: ajv
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>