Stoppable server

[rushsteve1]

Summary

Added the Server::stoppable() function which provides a method for gracefully stopping a Server.

Motivation

There was obvious way to gracefully stop a Rouille server, and I ended up implementing this code in my own project. Since it's fairly simple and a useful feature I figured I would contribute it back.

See the provided documentation example for basic usage.

[rushsteve1]

As @LPGhatguy pointed out it was consuming a lot of CPU. I fixed that with c1256a4

As it stands on my machine the system monitor reports 0% CPU usage when idle and ~50% when under heavy synthetic load.

The only caveat is that there is at most 1 second between checks to see if the stop has been requested. There is no delay between server responses however. I believe that 1s can be lowered but I thought it was a good conservative delay.

Linked here is a simple project with a synthetic benchmark for testing purposes.

[rushsteve1]

@tomaka Are there any further issues with this? If not I would like to get it merged.

[rushsteve1]

I just realized that this PR is a (better) alternative to #203

Also @tomaka again can I please get a response or something? This has been just sitting for a few weeks now.

[rushsteve1]

Bumping for visibility and @tomaka once again

[tomaka]

Well, I pushed a commit to your branch that updates it over master. But I screwed up by breaking the example, and now I don't have the authorization to push to your branch anymore. Sorry about that.

[rushsteve1]

I updated it to fix the test, but now some random macro breaks on the old version of Rust you have it testing against so the CI fails.