When dcb_close() is called, the DCB is only marked for closing
and the actual closing takes place only after all event handlers
have been called. That way, the state of the DCB will not change
during event processing but only after.
From a handler perspective this should now be just like it was
when the zombie queue was present.
TODO: There are far too many state variables or variables akin to
state variables - dcb_role, state, persistentstart, n_close -
in DCB. A cleanup is warranted.
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