partial table migration

[stoiev]

There are situations that migration can or need to be done partially.

On our specific case, we have some time series data that could be migrated using time frames (old and immutable data without service downtime, and hot data with service downtime).

But certainly there are more case, like garbage data that can be cleaned in migration.

[tarzanek]

lgtm, @stoiev I suppose you tested and used this code so I don't see any problem with it

[tarzanek]

merging, thank you @stoiev !