Diagnostic float: Record position before debounce time

[ilya-bobyr]

It seems that we should be recording the current buffer, cursor position and change tick before we wait to see if the float should be shown or not. Otherwise, if any of the above change during the g:lsp_diagnostics_float_delay our recording would be incorrect.

[ilya-bobyr]

This is a first commit in a sequence of 7 commits that improve the float behavior for me.
Mostly by fixing some subtle bugs.

Each commit is documented, should work, and can be reviewed independently.
I think it makes sense to look at all of them eventually.
If you prefer, I can create a single PR for all of them combined.
Or I can create new PRs one by one.
Posting the next commit only after the previous commit goes through the review process.

All the changes are in the same area, so they rely on each other quite a lot.
To some degree, some changes are justified by what comes next.
But I was trying to split them such that the future dependencies are rather weak.