GC objects in metadata chain based on height (epoch)

[carpawell]

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

We need to GC LOCK, TS, and REGULAR objects with expiration set. neo-go has a GC mechanism that is run every block approval; this should be reused, it should not be a separate chain call.

Describe the solution you'd like

1. Extend indices with expiration epoch (if any) from Feat/meta data native contract #3759
2. Adapt neo-go's expiration mechanism and drop object metadata based on this index (likely in batches, not more than N records in DB)

Describe alternatives you've considered

Separate routine triggered externally.

Additional context

#3759 (comment), smth should GC objects anyway.

[carpawell]

Likely, we need sidechain to know the current FS height. Or epochs should be converted to blocks, and it should be VUB part of metadata. Epoch handling leads to additional code on IR side. VUB approach leads to potential desync if epochs are delayed.