This addresses issue #7504 where a single weed mount FUSE instance
does not fully utilize node network bandwidth when reading large files.
Changes:
- Add -concurrentReaders mount option (default: 16) to control the
maximum number of parallel chunk fetches during read operations
- Implement parallel section reading in ChunkGroup.ReadDataAt() using
errgroup for better throughput when reading across multiple sections
- Enhance ReaderCache with MaybeCacheMany() to prefetch multiple chunks
ahead in parallel during sequential reads (now prefetches 4 chunks)
- Increase ReaderCache limit dynamically based on concurrentReaders
to support higher read parallelism
The bottleneck was that chunks were being read sequentially even when
they reside on different volume servers. By introducing parallel chunk
fetching, a single mount instance can now better saturate available
network bandwidth.
Fixes: #7504