Fix potential segfault when finally in a generator is run during shutdown

If a generator is destroyed in a finally block it will resume the generator to run that finally
block before freeing the generator. This was done in the object storage free handler.

Running user code in the free handler isn't safe though because the free handlers may be run
during request shutdown, already after several key components have been shut down.

This is avoided by doing the finally handling in the dtor handler. These handlers are run at the
start of the shutdown sequence.