- optimize use of `nimsuggest highlight`
- when Notepad++ is shutting down try to quit `nimsuggest` faster
- other minor improvements

Fix a regression in 515c38b (v1.2.3 build 6592a416).

In a rare case due to an optimization "highlight symbols" (on scroll) feature might be silently disabled in a Notepad++ tab, but not in other tabs.

Accept partial response from `nimsuggest` slow query.

For instance the "highlight" query may be too slow for very large files, but the partial response is still useful.

fix possible segfault when using new feature "highlight symbols" since v1.2.0