Speed up SAF 100x

[lmagyar]

Summary:

• When there are 1000s of files in a dir, even opening a file takes several minutes!!!
• The root cause is that creating even 1 SafFile involves a call to DocumentFile.findFile() and it calls listFiles(), and that is extremely slow on a big dir.
• I've copied the Cursor code from RoSafFileSystemView.getFile() (with some minor improvements).
• Now opening a file is a fraction of a second on a dir with 1000s of files!!!
• Though listing a dir is still slow, it uses listFiles(), and we can't do anything about it.

One issue with the code below:

• When there is an invalid path, like /valid-folder/nonexistent-folder/nonexistent-folder/file.xxx, what should we do with nonexistent-folders???
• We should throw an Exception, but this is not allowed here, we can't return null, it will harm SftpSubsystem.
• Original code returned a SafFile referencing /valid-folder/invalid-folder and finally a file was created with the name of invalid-folder, still not a good outcome. And without any error message.

UPDATE: this PR can remain as is, this issue will be fixed in a new PR, because it got quite complex

Possible better solutions:
- Extend FileSystemView.getFile() with a throws Exception and modify SftpSubsystem's code? I remember you don't like modifying the original server's code.
- Create some dummy "fails on everything" SafFile+SafFtpFile+SafSshFile, and that will throw the proper FileNotFoundException on the first call? Maybe this is the most analog to what would happen on a real file-system with File. But it smells like an ugly hack.
- Any idea???

[wolpi]

That sounds quite good 🤩
I've to take a deeper look, also together with the other PRs.

[lmagyar]

After seeing #205, I will experiment with the non-existent-path problem. Until it is done, I mark this as draft.

In case of plain old file-system, this is not an issue to create a file without creating a new folder, FsFile has a workaround for this here.

We should do something like FsFile in SafFile/RoSafFile, but it is hard, SafFile requires an existing parentDocument, and in this case it is nonexistent. More investigation is needed.

[lmagyar]

Without change, it's ready for review again.

This PR can remain as is, the "file under nonexistent folder issue" will be fixed in a new PR, because it got quite complex.

[wolpi]

Ok, the trick was to mix those 2 SAF APIs (DocumentFile and URL-based-API). That really improves it. And the changes are not at an end yet 😄