Implement #214 JUICE_CONCURRENCY_MODE_USER

Thanks for helping me figure this out @paullouisageneau. I think I've addressed most of the things discussed in the original issue. I also added a test too. There might be some things you want to change though so this PR is open to maintainer edits. Also some guesswork and copying-and-pasting was done, so there might be some dead code or things that don't make sense. And I think I have my tabs and spaces straight... although my sanity has been permanently lost

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I just made DNS resolution's defined behavior blocking in JUICE_CONCURRENCY_MODE_USER. There were some things that confused me about asynchronous DNS handling namely I think conn_impl can be NULL until DNS resolution is finished? I also just like the fact that it means there are no threads required when using this concurrency mode. I think it's reasonable to make DNS resolution the user's problem, but feel free to rewrite this part if you want non-blocking DNS resolution.
Generally the smallest way to use juice_user_poll is a little verbose. It is basically like recv(3) even to the point where juice_user_poll returns for all packets including those only used for ICE. This may have some practical use, but it's more of a side-effect of the implementation. It's also a bit disjoint since everything should still be handled in on_recv. This at least
```
int ret;
do {
    char buffer[1500];
    int ret = juice_user_poll(agent, buffer, sizeof(buffer));
} while (ret > 0);

if (ret < 0) {
    // Handle error
}
```
The benefit of pushing the loop out of juice_user_poll here rather than calling on_recv in the function over and over is to facilitate zero-copys which I give an example of in user.c.
Each agent has its own lock like we discussed in #214 so agent use across threads should be thread safe for everything except juice_destroy if I did everything correctly
The last condition in this expression seems like it shouldn't be needed, but I needed it to make the connection establish in a reasonable amount of time. I'm thinking there is something going wrong with how the timer is set, but it's a bit beyond my understanding to fix this.

paullouisageneau / libjuice

Implement #214 JUICE_CONCURRENCY_MODE_USER #226

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