Multiple data sequence numbers per commit

[SergeiPatiakin]

• introduce the concept of Data Sequence Number Group (DSN group), defined as the set of DataFiles with the same data sequence number
• allow a commit to consist of multiple DSN groups
• allow consumers to call transaction.new_data_sequence_number() to start a new DSN group

[gruuya]

Btw there will be a (minor) conflict here with JanKaul#227 or JanKaul#228, depending on which one gets merged.