re_datastore: support for batched row insertions

[teh-cmc]

While it is possible to insert any number of columns (== components) at once, we currently only support inserting a single row at a time.

While #584 actually lifts this limitation from a public API standpoint (i.e. the caller can now pass many rows of many components to be inserted), internally we are still splitting the batch into single rows and inserting those one by one.

We should think about being able to actually support zero-copy/zero-alloc batched insertions where possible.
In particular, this will need to integrate nicely with the recently added component bucket chunks.

This requires the flatter MsgBundles that have been talked about: we need MsgBundles to be able to carry more than one timepoint in order to insert more than one row!

[teh-cmc]

Superseded by #1619