This bug was also present in varnish-cache (will document) and found
its way into fellow through the api-definition-by-example paradigm of
varnish-cache:

In varnish-cache vbf_stp_error(), an existing storage object is freed
to be replaced with a new one, potentially from a different storage
(usually -sTransient).

In this case, ObjBocDone is not called, because the boc is to be
preserved - from the client's perspective, the backend transaction is
still in progress, and the client is waiting for the busy object to
finish.

At the storage level, however, the process of creation of a new object
_is_ finished, and any busy object state which the storage might keep
needs to be finalized.

So, if oc->boc is not NULL for an ObjFreeObj() call, the stevedore
needs to finish any busy object transaction.

Fixes #14