Stop queuing discovery operations

We need to be able to remove a folder or interrupt a discovery before it completes.

For instance, if a user misclick and starts indexing their 10TB NAS, it's probably not optimal to ask them to wait until the discovery is complete to cancel the operation.

• We still want to handle all operations from the discoverer thread, since interrupting a running discovery might block
• When a discovery/reload task is in progress, interrupt interrupt it to process the ban/removal request
• Once done, reschedule the previously running task (which imply that we have some kind of a current running task handling)
• We still need to ensure onDiscoveryStarted/onDiscoveryCompleted & onReloadStarted/onDiscoveryCompleted callbacks are still invoked properly even when we interrupt a discovery to process a ban/removal request