Issue #1796144: Added an event processor plugin type for repositories.
- Support multiple plugins by repository plugin slot.
- Support plugins acrooss different vcs's.
- Added a VersioncontrolPluginConfigurationInterface, which defines how
to configure a versioncontrol plugin.
- Better serialization for VersioncontrolEvent.
- Call processor plugins on hook_versioncontrol_code_arrival().
- Added a per backend configuration page for event processors.
- Declare and use a new queue to run repository event processors.
Thanks to sdboyer for the comments on the architecture behind.