Throttle feature: do not send too much requests to providers

Here's a first patch, it seems to work well for my needs.

Thank you very much itamair for the review. I'll answer your questions, but I just figured out that the patch I submitted does not contain the last changes I made. I think I messed up at some time!

Here's the correct patch. It contains two bug fixes I made that were mysteriously not in patch #2. It also contains your coding standards improvements from #4.

To answer your questions:
- Yes, it makes sense to me that the Nominatim Geocoder server provider setup/annotation definition already bundles throttle presets. Their doc says "an absolute maximum of 1 request per second" and "Make sure you stay well below the API usage limits.". So I put 1 request per 2 seconds. But I just figured out that I put it in the generic nominatim provider instead of the OSM one. I'll submit a new patch for this.
- About the parameters, for the OSM free service, Root URL is obviously required, but it should not change. OSM Nominatim asks for User agent or Referer, so it's required to fill at least one of the two. Personally I filled both and was not banned for that.

Whoops, here's the patch.

Here's the fix for the Generic Nominatim vs OSM Nominatim problem.

Hi @itamair, I think it's ready for commit if the review is OK.
About the composer problem: I guess it won't work if people update without using composer. A hook_requirements() implementation may help to warn people? But if they use composer update drupal/geocoder, I guess the dependency will be downloaded automatically by Composer, no? I'm not a composer expert, I'm not able to confirm this without being able to test it.

As I could not commit to drupal/geocoder to test, I cloned the official repo as "insite/geocoder" in our own composer repo, and required it in a project. Then I committed the patch to this clone and did:

[...]$ ls vendor/davedevelopment
ls: impossible d'accéder à 'vendor/davedevelopment': Aucun fichier ou dossier de ce type
[...]$ composer update insite/geocoder
Gathering patches for root package.
> DrupalProject\composer\ScriptHandler::checkComposerVersion
Loading composer repositories with package information
Updating dependencies (including require-dev)
Package operations: 1 install, 1 update, 0 removals
Gathering patches for root package.
Gathering patches for dependencies. This might take a minute.
  - Installing davedevelopment/stiphle (0.9.2): Loading from cache
  - Updating insite/geocoder dev-master (7e01579 => 64693c1):  Checking out 64693c19e9
davedevelopment/stiphle suggests installing predis/predis (~1.1)
Writing lock file
Generating autoload files
[...]$ ls vendor/davedevelopment
stiphle
[...]$ 

So, if people use composer to update geocoder to a newer version/commit, they will get the dependency automagically*.

There is a problem if people do not use composer (until #2622420: Site-builders should be able to use projects with composer dependencies is done). So I started writing a hook_requirements() implementation. But I remembered that composer is already a requirement according to the module documentation, and there is for example no check to see if Geocoder-PHP is installed, whereas it's a composer dependency. We might add that kind of check for people trying to use the module without Composer, but that's another issue, no?

*This is not the case if they simply apply the patch using cweagans/composer-patches, as the patch is applied AFTER composer has computed the dependencies.