Metadata fetched may be inconsistent

[Lorak-mmk]

Fetching metadata requires multiple queries. If there is a schema change between those queries then we'll get inconsistent view of the metadata.
While this problem should be very rare, and in current implementation new metadata will be fetched after 60s anyway it's not that important, #1008 may make this problem worse.
The solution I see is to fetch schema version first, then fetch metadata and then fetch schema version again. If schema version didn't change then we have consistent view of metadata. If it did change then we can re-fetch the schema.