Use the hub injected in the constructor of the listeners rather than retrieving it from the SDK singleton

[ste93cry]

This PR refactorizes the event listeners to make them use the Hub injected into the constructor rather than getting it using the global functions. This eases the unit testing of those classes. Note that by default the injected hub is under the hood an instance of HubAdapter and thus it works exactly as before by proxing each call to the current hub that is retrieved on-the-fly

[ste93cry]

@Jean85 do we really need to make the priority of the listeners configurable? I know it's a really small feature and doesn't have a big impact in the maintenance of the code, but I don't think it's useful since people can simply adjust the priority of their listeners

[Jean85]

Listeners need to run after certain stuff, making them configurable makes the installation a lot easier. Requiring users to change their code to install ours is not fine IMHO.

What issues do you have with those?

[ste93cry]

No issue at all, I was simply questioning because I never heard anyone needing this and also trying to look around GitHub using the search feature to see who does the same I didn't find many results. I'm totally fine with leaving this as-is if you think it's worth keeping the feature though, I have some doubts about the usefulness but it's just my opinion

[Jean85]

The issue was requested in #49 the first time, so there's definitely a need.

[ste93cry]

Looking at that PR I think that the main issue was that the listeners at that time were all using the default priority, so people could not inject their own listeners between the Symfony ones (if there was any) and the Sentry ones. You decided to solve the issue by making the priority configurable, which is a way to solve the issue. The other way (it would had required a new major version of course to avoid breaking BC) was to explicitly set the priority. Both ways are fine, as I said I don't really have any issue with keeping things as they are now 😃