Replicated system tables

[Nikita Batrak]

Hello!

We use a configuration with mirroring data for HA/disaster recovery purposes. It requires replication of all our tables. But system tables use ReplacingMergeTree for schema_migrations and MergeTree for ar_internal_metadata.
It's impossible to change it right now other than monkey-patching.
I want to create a PR, but it would be great if core contributors would write their vision of this feature (how to configure it, etc.).

Thanks!