Increase checksum buffer to 128kb, improving download performance.

[stewartsmith]

Reading 2kb at a time to compute the checksum limits network throughput. Bumping up to 128kb seems to give a good balance of memory usage and performance.

Benchmarks done on a m5n.16xlarge EC2 instance doing a reposync on the Amazon Linux 2023 x86-64 repositories showed that this change, when combined with the (smaller) benefits of my avoiding libc IO patch, reduce system CPU time by another half second, and cut a further 3 seconds off total time:

102s (original) -> 99 (no libc buffered io) -> 95s (this patch)

[stewartsmith]

For reference, my benchmarking has been done on a m5n.16xlarge EC2 instance to the in-region S3 buckets as well as to the CDN repositories. That instance type has 256GB memory, a 75Gbit network connection, and is a 64 core Cascade Lake system. The root volume is a 256GB gp3 EBS volume with 500MB/sec of IO and 3000 IOPs.

The background of this is that a lot of EC2 instances don't live that long (relatively speaking), and never install RPMs except on launch - so all the time-to-install RPMs is time spent scaling up a system that could be better served by running the customer workload.

Goes well when paired with #294