DataShard: followers stop syncing from the Sys table

[snaury]

There's a long standing issue where followers start rejecting all queries complaining about mismatching path owner id (which is equal to the invalid value), while leader's Sys table has the correct value. We currently rely on the table's Serial counter to cache these values. It appears however that Serial is only updated on mem table updates, and after the table is compacted without any newer updates, the Serial field will remain zero at a rebooted follower, which is treated as "no changes in the table". We need to use a more complex comparison that disambiguates these cases using the table's epoch.