Remove serialization non C41 constructors from stats

[robertbindar]

With this change in, PR #4685 and others to follow should:

a) remove Subarray::set_stats and respective call from serialization/query.cc b) where Subarray is constructed in serialization/query.cc`, change constructor call with the following code snippet:

auto &stats_data = stats_from_capnp(reader.getStats());
Subarray subarray(array, layout, query_stats, stats_data, dummy_logger, true);

c) The constructor calls parent_stats->create_child(prefix, stats_data); d) When all migrations are done, make Stats::populate_with_data private

---

TYPE: NO_HISTORY
DESC: Remove serialization non C41 constructors from stats

[shortcut-integration]

This pull request has been linked to Shortcut Story #40316: Remove serialization non C.41 constructors from stats..

[KiterLuc]

As discussed offline, let's try to make the stats() accessors of all class return a const Stats& object. Let's also validate that the stats we get from REST are identical before and after this change.

[robertbindar]

Manually tested this in a local REST server and stats are identical before and after the change.
We're stuck with Stats* Query::stats() and Stats* StrategyBase::stats() because due to the current architecture, the stats objects passed from serialization end up calling Stats::create_child which cannot be const.