To avoid reactor stalls during schema upgrades of memtable and cache entries,
we want to do them interruptibly, not atomically. To achieve that, we want
to reuse the existing gentle version merging mechanism. If we generalize
version merging algorithms to handle `mutation_partition`s with different
schemas, a schema upgrade will boil down simply to adding a new empty MVCC
version with the new schema.
In a previous patch, we already generalized the cursor to upgrade rows
on the fly when reading.
But we still have to generalize the other MVCC algorithm: the merging of
superfluous mutation_partition_v2 objects. This patch modifies the two-version
merging algorithm: apply_monotonically(). The next patch will update its caller,
merge_partition_versions() to make of use the updated apply_monotonically()
properly.
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