Increase the prefetcher's low water mark so it restarts fetching data earlier.

    enum {

    enum {

        kPageSize            = 65536,

        kPageSize            = 65536,

        kHighWaterThreshold  = 5 * 1024 * 1024,

        kHighWaterThreshold  = 5 * 1024 * 1024,

        kLowWaterThreshold   = 512 * 1024,

        kLowWaterThreshold   = 1024 * 1024,

        // Read data after a 15 sec timeout whether we're actively

        // Read data after a 15 sec timeout whether we're actively

        // fetching or not.

        // fetching or not.