VSLQ_Flush() vtx_force()s all incomplete transactions to be ready and calls
vslq_process_ready().

vslq_process_ready() retires vtxes until the callback returns with non zero,
so if our dispatch callback does return with an error, flushing will stop,
so we may end up with a non-empty ready list in vslq after calling flush
just once.

To ensure that we flushed everything and vtx_retire()d all vtxes
(which may contain refs to chunk_t_bufs living in malloc()ed memory),
we need to keep on flushing until we know the bowl is empty.