Fixes #2946 in the sense that we want to treat backend synth like any
other object and kill the stale object it replaces.

The two HSH_Kill calls could also be subsumed in vbf_fetch_thread(),
but after an irc discussion, phk decided that he wanted to aim for even
more consolidation later.

The test checks that we remove the stale_oc, but not when abandoning the
bereq.