S3: Add global_prefix and prefix_folders options

[nvaatstra]

Adds 2 configuration options to simpleblob, to offer extra control over handling prefixes within a bucket:

• global_prefix is a prefix applied to all operations, allowing work within a prefix seamlessly (Default: "" - matches current v0.2.1 behaviour)
• prefix_folders can be enabled to prevent List operations from including all items in subfolders (Default: false - differs from v0.2.1 behaviour, but this is not expected to affect anyone)

Tests have been updated to cover both options.

Examples - Recursive

(Some items below refer to Recursive, which is the same as !PrefixFolders. We renamed it)

If a bucket contains the following:

- bar1
- bar2
- foo/bar3
- foo/bar4

Then a List() using Recursive unset or false will yield (this is the current v0.2.1 behaviour):

- bar1
- bar2
- foo/

If Recursive is set to true, then a List() will yield:

- bar1
- bar2
- foo/bar3
- foo/bar4

Examples - GlobalPrefix
If a bucket contains the following:

- bar1
- bar2
- foo/bar3
- foo/bar4

Then a List() using GlobalPrefix unset or "" will yield (this is the current v0.2.1 behaviour):

- bar1
- bar2
- foo/

If GlobalPrefix is set to "foo/", then a List() will yield:

- bar3
- bar4

And a Store() action for name bar5, followed by List() will yield:

- bar3
- bar4
- bar5

The actual contents of the bucket will now be:

- bar1
- bar2
- foo/bar3
- foo/bar4
- foo/bar5

Examples - GlobalPrefix & Recursive
global_prefix = "foo/" and recursive = true will result in a List() returning:

- foo/bar3
- foo/bar4
- foo/bar5

global_prefix = "" and recursive = true will result in a List() returning:

- bar1
- bar2
- foo/bar3
- foo/bar4
- foo/bar5

[wojas]

I would like to make listing all filenames with any slashes the default, instead of requiring a per-backend opt-in.

So instead of Recursive we would provide an option to opt-out of the recursion,

[nvaatstra]

Below commit removes Recursive and replaces it with PrefixFolders.

PrefixFolders can be enabled to make List operations show nested prefixes as folders instead of recursively listing all contents of nested prefixes.

Tests updated accordingly.

Examples - PrefixFolders
If a bucket contains the following:

- bar1
- bar2
- foo/bar3
- foo/bar4

Then a List() using PrefixFolders unset or false will yield:

- bar1
- bar2
- foo/bar3
- foo/bar4

If PrefixFolders is set to true, then a List() will yield:

- bar1
- bar2
- foo/