Support client 2.0

[Jean85]

The development of the Sentry client 2.0 is currently under development: getsentry/sentry-php#670

The new version is a complete overhaul, and includes the new Unified API: https://docs.sentry.io/clientdev/unified-api/

This means a lot of changes (and simplifications) on this side, with the Symfony integration: we could drop basically everything, make the main hub available in the container and document the usage of the general static function for general, on spot reporting.

Progress needed:

• require the 2.0 client SDK metapackage (Use SDK metapackage #190)
• drop the exception listener
• call init() somewhere during app startup
• add some integration tests if possible (spin up a basic SF app and try to trigger some errors?)
• document the possibility of using the Unified API static functions
• inject in the client a reasonable entry point (the hub?)
• replace the user auto detection with some different automation
• completely refactor the configuration: try to reduce the available options to the bare minimum
• update the Flex Recipe (Add different recipe for Sentry bundle v3 symfony/recipes-contrib#621)

[Jean85]

Since only one issue remains, I've split it out in #209.