Decomposed based posix filesystem driver

[aduffeck]

This PR adds a new posix fs driver which stores files and metadata directly on a posix compliant directory. It also adds some special handling for gpfs, i.e. for improving the notifications about changes to the underlying data.

It is based on decomposedfs but uses a copy of the lib with some changes that require some refactoring before it can be merged back to the "real" decomposedfs code. We should consider doing this before merging the pr in order to avoid code cuplication.

Supported Features:

• Personal and project spaces
• Basic file operations, sharings, previews, etc.
• "Assimilation" of new data being added to the filesystem bypassing ocis (using either inotifywait, gpfs file audit logs or gpfs watchfolder queues)
• Handling of changes being made to the files on disk (renaming, moving, etc.)
• gid mapping (new files/directories will be created with the gid of the space root)

Currently missing:

• Trashbin support
• File versioning

Ocis companion PR: owncloud/ocis#8598

[update-docs]

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[butonic]

tested with redis started and inotifywait installed using this ocis config:

					"STORAGE_USERS_DRIVER": "posix",
					"STORAGE_USERS_POSIX_ROOT": "/home/jfd/ocis-posix",
					"STORAGE_USERS_ID_CACHE_STORE": "redis",
					"STORAGE_USERS_ID_CACHE_STORE_NODES": "127.0.0.1:6379",

problems noted in owncloud/ocis#8598 (comment)

[butonic]

no blockers from my side