Merge branch 'faster' into 'master'

Faster IO500

Add some optimizations.
1. With write inlined, mdtest inneficciently gets all the 4k data for all the request. A stringview optimizes the feature.
2. batching mmetadata request at the client (LIBGKFS_METADATA_BATCH=ON/OFF and LIBGKFS_METADATA_BATCH_THRESHOLD=64).

See merge request !305