When removing a plugin-insert that has a sidechain on a bus
that is implicitly soloed the following happens:
* Route::remove_processor() takes a WriterLock(_processor_lock)
* The sidechain input is disconnected
* Route::sidechain_change_handler() is called which calls
* Route::input_change_handler()
* Since the route is implicitly soloed, propagation is attempted
which calls
* Route::direct_feeds_according_to_reality()
which takes ReaderLock(_processor_lock)
Recursive locks, reader-lock after writer-lock don't cause
a deadlock, releasing the reader-lock effectively also
releases the writer-lock...
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