In #6478, we stopped passing through Redis errors to the top-level
Responder object, preferring instead to live-sign. As part of that
change, we logged the Redis errors so they wouldn't disappear. However,
the sample rate for those errors was hard coded to 1-in-1000, instead of
following the LogSampleRate configured in the JSON.
This adds a field to redisSource for logSampleRate, and passes it
through from the JSON config in ocsp-responder/main.go.
Part of #7091