Gmoshkin/minor fixes

Summary

• refactor: mod vshard

• debug: log more info

• test: add tests to check instance & replicaset structs match their space formats

• refactor: cleanup replicaset weight stuff

• fix: reduce number of idle governor loop iterations

• test: don't use explicit system space ids

Close #...

Ensure that

• New code is covered by tests
• API is documented
• Changelog is up to date
• (if Lua API changed) Lua API version is bumped in luamod.rs
• (if necessary) A follow-up doc issue is created in picodata/docs and linked here