datastore: serialize back into a stream of MsgBundles

[teh-cmc]

This PR adds DataStore::as_msg_bundles(), which serializes a DataStore back into a stream of MsgBundles that is functionally equivalent to the original stream it was built from.

Some shortcuts are taken, which is why the output stream is functionally equivalent but not yet identical to the original input stream: in particular, autogenerated cluster keys are dumped as if they were user-defined. That's for another PR.

This gets us most of the way towards #1394, although we'll still need another PR to integrate it all into the save-to-file logic.

This also fixes a nasty instability issue when sorting the dataframes used for testing, which cost me some hair while writing this and might possibly be the reason for the flaky gc_correct test we've seen for a while.

[teh-cmc]

Just realized we'll want to be able to optionally specify a time range ultimately (e.g. for saving a selection); this is trivial to do at this point and will come in due time in another PR.

[teh-cmc]

Putting this on hold for now, see #1535 for rationale.

[teh-cmc]

Whether this is on hold or not, i need to backport the changes to store_polars.rs into main; this is very likely the fix for #894.

[teh-cmc]

Fixed by #1535, which went the other route instead (#1397), for the reasons explained in detail in #1535.

The fixes related to sorting dataframes are backported via #1549.