Start using FlatBuffers instead of Google Protobuf

[kornilova203]

Start using Cap'n Proto instead of Google Protobuf for client-server communications. This will speed up visualization and reduce memory consumption.

[kornilova203]

I could not find supported stable cap'n proto library for browser javascript:
Here are all options I could find:

• capnproto/node-capnp only for node.js
• jscheid/capnproto-js outdated
• jdiaz5513/capnp-ts unstable + it's typescript version, so it will add extra complexity to the plugin

Next option to check is FlatBuffers

[kornilova203]

FlatBuffers is much faster.
It takes ~800ms to create and serialize a tree with 524288 nodes with flatbuffers, and ~3s with protobuf.
Although resulting file is bigger 50Mb vs 20Mb. But disk space should not be a problem and serialization/deserialization time is much more important.
Also FlatBuffers should reduce runtime memory consumption.