Memory use: Use ref-counted buffers for images and meshes

[emilk]

We currently store all incoming log messages (so we can save them as .rrd files) while also indexing and storing their contents in the data store. This basically duplicate all data.

We should make sure that big components, such as meshes and tensors, use Arc under the hood so that the log messages and the store can share the memory.

Currently we use Vec when storing the contents of an image (TensorData::U8) and a mesh (RawMesh3D).

We want to use ref-counted types like Arc or arrow2::Buffer, otherwise we duplicate our memory use.

The natural thing to use is arrow2::Buffer<u8> or Arc<[u8], but neither of those implement ArrowSerialize/ArrowDeserialize yet (see below).

See also:

• Add a test of memory use when logging a lot of big images #1372
• Can't (de)serialize Buffer<u8>> DataEngineeringLabs/arrow2-convert#101
• Implement ArrowSerialize + ArrowDeserialize for Arc DataEngineeringLabs/arrow2-convert#102

An alternative solution is to not store LogMsges at all and instead the data store to file.

[nikolausWest]

While I definitely agree with these improvements I think this issue also points to a higher level question: should we really be keeping all the incoming log messages in memory at all?

First of all we should probably make it easier to set up ways to stream messages directly to a file, both from the SDK and as it hits the viewer / server.

Still, it's very useful to be able to decide to save a recording (.rrd file) after the fact. Shouldn't we be able to to quite simply serialize and deserialize the data store to achieve the same ends?

[teh-cmc]

Fixed:

• We don't store data LogMsgs anymore.
• TensorData, EndcodedMesh3D and RawMesh3D all use Buffers under the hood.