[js worker] Scheduling and interruption problems

[ejgallego]

The web worker is not scheduling didChange / interruptions events properly.

The visible effects on the UI is that the Web Version of coq-lsp starts to lag.

This requires more investigation, as it makes coq-lsp unusable quickly. We got it running fine in the past (first versions of jsCoq for example), so I'm optimistic it is a bug we have introduced in later refactorings.