Make max-readahead configurable to enhance read performance

[ddrid]

Currently, readAheadMax is a non-configurable parameter for fuse users, set by default to conf.Chunk.BlockSize * 8, typically 32MB. This isn't sufficient for low-concurrency read requests. I intend to make max-readahead a user-configurable parameter.

After testing, I set max-readahead to 512MB and observed that under single-threaded, with uncached local disk reads, the throughput increased from 230MB/s to 2.2GB/s, showing a 10x improvement.

[solracsf]

Thanks for the PR. Maybe also document it (in docs) too?

[davies]

we can provide --max-readahead to overwrite this default setting.