Kill linters on BufWinLeave if g:ale_lint_on_enter is 1

If `g:ale_lint_on_enter` is set to `1`, the default, ALE will run the applicable linters on a buffer when the `BufWinEnter` and `BufRead` events fire. Quickly switching between buffers for files that have slow, heavy linters attached (e.g. gometalinter) will result in those linters being spun up concurrently for each buffer. Meanwhile, one implication of `g:ale_lint_on_enter` being set to `1` is that, when switching to a buffer, the loclist will be cleared and repopulated. This means that the output of linters on non-active buffers is never actually used.

The upshot of all this is that it should be safe to kill any running linters on `BufWinLeave` if `g:ale_lint_on_enter` is `1`. This would have the benefit of reducing CPU load and battery use in this scenario.

Start by tracing ALE's handlers for BufWinLeave, BufWinEnter, and BufRead, along with the g:ale_lint_on_enter setting. Confirm how running linters are tracked and verify that, when the setting is 1, leaving a window stops those linters without disrupting linting for the active buffer.

Written by the indexing model from the issue text.