Save .rrd from store instead of saving LogMsg:es.

[emilk]

Currently the viewer stores all LogMsg:ed it receives in order to be able to save them as a .rrd file.

This leads to a lot of extra memory user, often at 2x or more, since the same data is also stored in the data store.

We need to support:

• Saving the entire store
• Saving a time-range of one timeline

We should implement batch ingestion before we work on this, as it affects how data is stored in an .rrd file.

Blocked on:

• re_datastore: support for batched row insertions #589
• Batch data ingestion #1395

[teh-cmc]

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