Store time at which metabase was resynced

[carpawell]

Is your feature request related to a problem? Please describe.

I'm always frustrated when we miss some details about meta when it behaves badly.

Describe the solution you'd like

Store some timestamp that indicates the last metabase resynchronization. Do not think absolute time is a good idea, so epoch can be a better option.

Describe alternatives you've considered

(:

Additional context

Every time we debug metabase is an "additional context".

[roman-khimov]

We'll change metabase format a bit in the next version, so we better do this at the same time.