Instead of always creating the trust anchor timer (dns_nta_t) on the
main loop, create the timer on the current loop and associate each
dns_nta_t object to the loop it was created on. This simplifies the
timer handling as everything is run on the associated loop.
During the change, the dns_nta_t structure was renamed to dns__nta_t
and changed to be fully internal to the nta.c compilation unit, and the
dns_ntatable_t structure was made opaque. This required no change to
code using the API as dns_nta_t never had any external users and the
dns_ntatable_t was properly accessed only by using function calls.
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