Do not call startWorker every 100ms on typing

When smth is typed into the CodeMirror editor
it schedules a job to rehighlight the next 500 lines of the document. This job
is intended to be executed in a `400ms` time (as hardcoded in
`makeChangeSingleDocInEditor` method), but actually
400ms delay gets overwritten with `100ms` delay which is set
from `updateDisplayInner` method.
The problem with this is that people hardly ever type 10 chars/second,
so the highlighter gets its time to rehighlight 500 lines of the
document while keydown events get postponed and user might feel some
slowleness.