The cache lookup was performed before rule matching, using the caller's
strategy (usually AsIS/0) instead of the resolved strategy. This caused
cache misses when ipv4_only was configured globally but the cache lookup
expected both A and AAAA records.
Remove LookupCache and ExchangeCache from Router, as the cache checks
inside client.Lookup and client.Exchange already handle caching correctly
after rule matching with the proper strategy and transport.
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