Improve ALEFix performance for neovim (#3974)

* Avoid performance problems with setbufline() and Treesitter

Call nvim_buf_set_lines() instead.

Since this is a performance problem only in Neovim (Treesitter is only
available there), it doesn't matter that this API is unavailable in Vim.

Note: nvim_buf_set_lines() returns E5555, when set nomodifiable is on.

Fixes #3669

* Avoid sign flickering

The signs flickered because nvim_buf_set_lines() removes all signs from
lines that it touches, which will immediately be readded by Ale (causing
the brief flicker). This is intended behaviour in neovim [0].

Neovim itself faced this problem in their own LSP formatting sync,
although they had the problem with marks instead of signs [1].
Similar to how neovim fixed it by storing and restoring the marks [2],
we can do the same thing with signs.

In fact it is easier with signs, because sign_placelist() will just
ignore and skip invalid line numbers, so we don't need to filter signs
that are not valid anymore.

[0] https://github.com/neovim/neovim/issues/10880#issuecomment-526466042
[1] https://github.com/neovim/neovim/issues/14307
[2] https://github.com/neovim/neovim/pull/14630

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