lsp: Optimize diagnostics handling

Currently if a server sends a lot of diagnostics Kate becomes close to
unsable. One such server is the dart-analyzer which for some reason is
very eager about scanning every possible thing it can find in whatever
directory you are.

This patch includes some optimizations which makes the handling of
incoming diagnostics a lot faster.

Also, cache the KTextEditor::Attributes and reuse them so that kate
doesn't eat up too much memory in case a doc has a lot of diagnostics.

However, if you have a lot of diagnostics in the model and you close /
switch docs, it's still very super slow. Removing diagnostics from the
model file by file is not optimal at all. This needs to handled better.

(cherry picked from commit 32585d18)