Fix idle timeout for connected TCP sockets
When named acting as a resolver connects to an authoritative server over TCP, it sets the idle timeout for that connection to 20 seconds. This fixed timeout was picked back when the default processing timeout for each client query was hardcoded to 30 seconds. Commit 000a8970 made this processing timeout configurable through "resolver-query-timeout" and decreased its default value to 10 seconds, but the idle TCP timeout was not adjusted to reflect that change. As a result, with the current defaults in effect, a single hung TCP connection will consistently cause the resolution process for a given query to time out. Set the idle timeout for connected TCP sockets to half of the client query processing timeout configured for a resolver. This allows named to handle hung TCP connections more robustly and prevents the timeout mismatch issue from resurfacing in the future if the default is ever changed again.
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