proxy.config.http.cache.open_read_retry_time default value impractical

The default value for proxy.config.http.cache.open_read_retry_time is 10ms.

While running synthetic load testing simulating a thundering herd against a rotating manifest file I added debug into HttpCacheSM to printout time when read retries occurred. On a broadwell xeon with nvme drives I was consistently seeing more than 900ms between these calls. However under these conditions I was seeing at most 2-3 calls being made before the read lock was acquired.