Fix this issue follow these two points:
1. Keep track of how many queries are currently open. If `setServers()`
is called while there are open queries, error out.
2. For `Resolver` instances, use option 1. For dns.setServers(), just
create a fresh new default channel every time it is called, and then
set its servers list.
PR-URL: https://github.com/nodejs/node/pull/14891
Fixes: https://github.com/nodejs/node/issues/14734
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
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