Support for file locking via env

Some apps can be considered as GENERATORs so any file opened should not be accessible from another app considered as CONSUMER

We can achieve it using metadata only, and via two environment variables

LIBGKFS_PROTECT_FILES_GENERATOR

and

LIBGKFS_PROTECT_FILES_CONSUMER

This feature is used with FTIO, so we don't start a stage-out process if the file is still opened by some client app