rpc/keyserver: log lookups only once in 100 times
rpc/keyserver: log lookups only once in 100 times

Reduces load on the logger API, which increases server throughput.

No new lint or vet errors.

Change-Id: Id51d3f4678dddf721de69cfefda52d12ff32642b
Reviewed-on: https://upspin-review.googlesource.com/7706
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@google.com>

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