Khepri migration: match expected records

[dcorbacho]

During metadata store migration, Mnesia might generate messages for Table-Key pairs for in-flight writes.
These are skipped by mnesia-khepri-migration, as do not contain enough info to copy the record, but we ensure here that none comes through and we try to process it.

The event with Table-Record is generated at the time of write, which is properly processed after the initial copy of data.

Requires khepri_mnesia_migration 0.2.1

Types of Changes

• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI

Checklist

• I have read the CONTRIBUTING.md document
• I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• I have added tests that prove my fix is effective or that my feature works
• All tests pass locally with my changes
• If relevant, I have added necessary documentation to https://github.com/rabbitmq/rabbitmq-website
• If relevant, I have added this change to the first version(s) in release-notes that I expect to introduce it

[dumbbell]

Looks good to me! Thank you!