Previously, the zone loading and dumping was effectively serialized by
the dns_io_t mechanism. In theory, more IO operations could be run in
parallel, but the zone manager .iolimit was set to 1 and never increased
as dns_zonemgr_setiolimit() was never ever called.
As the dns_master asynchronous load and dump was already offloaded to
non-worker threads with isc_work mechanism, drop the whole dns_io_t
and just rely on the isc_work to do the load and dump scheduling.
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