Attachment directory needs to be hashed to avoid filesystem limitations

Confluence stores all attachments in a as subdirectories the data/attachments folder, the name of the subdirectory is the page id. This is bad for the following reasons:

• On linux few filesystems can handle directories with more than 2^15 sub directories. Once that limit is reached, new directories will fail to be created. The limit for ext3, the default file system for common linux distros is 31998 directory entries. More esoteric filesystems may raise this limit (please, not filesystem wars, ext2/3 is the most common filesystem) but it is still not a practice to be emulated.
• Traversing a directory this size slows file access and most tools are not designed to handle directories that large

This will become an issue in confluence hosted very soon.

The attachment directory should be hashed, similar to the way squid hashes its cache directory.