Make it possible to serve built pages while the site is reloading

[choldgraf]

Currently, when a re-build is happening, it is not possible to serve any of the already-built HTML pages from the last build. This is a pain because it can be useful to continue browsing your docs preview while a build is happening.

[asmeurer]

If not run asynchronously it would be nice if it at least queued up requests, so that if you click a link on the built site it eventually loaded it. Right now if you click a link while the build is happening nothing happens, and you have to wait for the build to happen and click the link again.

[AA-Turner]

Done in fda9582.

A

[asmeurer]

I'll need to test it out on the SymPy docs to see how things are now. I did actually notice that at some point in the past months either Sphinx itself or sphinx-autobuild have become faster, because a different much smaller site I maintain now rebuilds basically instantly, whereas it used to take a few seconds. But I haven't worked on the SymPy docs in a while so I'm not sure if #136 is fixed.