mvcc: Implement partition_entry::upgrade() using squashed()
mvcc: Implement partition_entry::upgrade() using squashed()

To reduce duplication of version merging logic.

scylladb/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,duarten/scylla,avikivity/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla