[CASSANDRA-21524][trunk] Row merging logic optimizations

[netudima]

there is no need to add empty interators to columnDataIterators if a row is null, also 2 loops over rows can be combined into one
MergeIterator logic is generic and can be used in many different use cases, for example to merge partitions and merge rows. While it is useful from development point of view to avoid duplicated code - it is non-friendly for JIT, we have polluted profiles with more that 2 types per call site, so we pay for it with frequent metamorphic calls, it is especially costly when we iterate over cells. A possible way to avoid this issue is to copy the class during a build time to create a separate class with exactly the same code but a different name. It allows to avoid development overheads with duplicated code to edit and at the same time helps JIT to optimize.
When we created a merged row we need to calculated minDeletionTime. If none of the merged rows had any deletion or expiring data, neither does the result, so we can pass the already-known min local deletion time and avoid rescanning the whole btree to recompute it.
DeletionTime.deletes check can skip Cell.timestamp invocation (potentially megamorphic) if the DeletionTime is LIVE.
In most cases merged cells have identical types, so we can add a fast path for it to Row.Merger.ColumnDataReducer#useColumnMetadata
ColumnDataReducer.versions can be switched from List to array to reduce allocations and to avoid extra deferences and checks/conditions.
replaceAndSink is large but we can split it to hot and colder parts to allow it to inline hot part into reduce method