Adding write_size cache, enable by config and disable via `LIBGKFS_DISABLE_WRITE_SIZE_CACHE=on`. Flush happens on close/fsync

This avoids heavy metadata traffic during small I/O especially for single shared files which can overwhelm the corresponding metadata daemon

Renamed Cache to DentryCache. Added several options to config.hpp Removed `LIBGKFS_ENABLE_DIR_CACHE`, added `LIBGKFS_DISABLE_DENTRY_CACHE` instead.

Env variable overrides config.hpp

This uses the extended dir RPC call.