process feedback in batches (fix #878)

[gares]

Before merging this:

• wait for merge of [new release] sel (0.6.0) ocaml/opam-repository#27433 in opam repo
• nix?
• bump sel version in https://github.com/coq/coq/blob/542ffa686f53f5cc429421613ac54347fbfacc5f/dev/ci/docker/edge_ubuntu/Dockerfile#L55 so not to break Coq's CI

[gares]

This fix makes Search list very quick to report the diagnostics to vscode.

Still, the updates to the goal windows takes a few seconds. It seems the time is spend in vscoqtop and not in vscode, so it should not be the rendering, but rather preparing/communicating to the web app the results.

It is much faster than before, but still perfectible.