[RFC] remove gettimeofday() usage

Ping @equalsraf. When this is merged, you'll probably be able to excise a hack.
I really would like to note these subtle improvements in [the wiki](https://github.com/neovim/neovim/wiki/Differences-from-vim) so that they don't get lost in time like tears in the rain. But I'm struggling with the "elevator pitch", ie I can't think of a simple one-sentence description of how this improves Vim for the end-user. 

Likewise for #962 #839 #850 #807.