Fix a race condition in xfrin_recv_done() when calling xfrin_reset()

Fix a race condition in xfrin_recv_done() when calling xfrin_reset()

When the xfrin_recv_done() function decides to retry the transfer
using AXFR because of a previous error, it calls the xfrin_reset()
function which calls dns_db_closeversion() on 'xfr->ver'. The problem
is that the ixfr processing of a previous message could be still
in process in a worker thread, which then can use freed 'xfr->ver'.

If there is an ongoing worker thread delay the AXFR retry until after
the worker thread has finished its work.