The cache deduplication in Client.Exchange uses a channel-based lock
per DNS question. Waiting goroutines blocked on <-cond without context
awareness, causing them to accumulate indefinitely when the owning
goroutine's transport call stalls. Add select on ctx.Done() so waiters
respect context cancellation and timeouts.
The FlatMap calls pre-populated routeAddressSet and routeExcludeAddressSet
before the for-loops which appended the same IP sets again, doubling every
entry. On Windows this caused CreateIpForwardEntry2 to return
ERROR_OBJECT_ALREADY_EXISTS.
Fixes#3725
Previously, the buffer was not reset within the response loop. If a packet
handle failed or completed, the buffer retained its state. Specifically,
if `ReadPacketFrom` returned `io.ErrShortBuffer`, the error was ignored
via `continue`, but the buffer remained full. This caused the next
read attempt to immediately fail with the same error, creating a tight
busy-wait loop that consumed 100% CPU.
Validates `buffer.Reset()` is called at the start of each iteration to
ensure a clean state for 'ReadPacketFrom'.
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.
