[ext storage] Consider moving to Flysystem

[PVince81]

Flysystem is a library that can take care of abstracting external storage: http://flysystem.thephpleague.com/
https://github.com/thephpleague/flysystem

This way we don't need to maintain our own implementation of the connectors, but can still contribute upstream.

As you can see on the website it supports the following:

• Local
• AWS S3
• Rackspace
• Dropbox
• Ftp
• SFTP
• Zip (??)
• WebDAV

To evaluate:

• how suitable it is for our use case
• how much work is needed to fit this in
• how its call/data caching works, whether we can use it as is or whether it duplicates what ownCloud's caching
• whether it can properly detect updates done directly on the remote server (which is a common issue)

One problem is that it requires PHP >= 5.4 (according to composer https://github.com/thephpleague/flysystem/blob/master/composer.json#L16)
So as long as we need to support PHP 5.3 this won't be a solution.

@icewind1991 suggested that we could use it for the backends it needs and for everything else we can use the older mechanism.

@DeepDiver1975 @icewind1991 @schiesbn @LukasReschke @karlitschek @MTRichards @craigpg

[PVince81]

And obviously when evaluating an external library:

• whether it's well maintained / active project

[LukasReschke]

I'd really appreciate this 👍

[th3fallen]

i showed this to lukas a few weeks ago or months (time flys) i think it would be an amazing improvement. 👍

[PVince81]

• evaluate the performance: whether the lib is doing things we don't need (I see it has events!) that could slow down the performance
• does the lib also support upload/download streaming instead of temp files ? (a feature we'd like to add)

[PVince81]

Flysystem was merged in core: #16923