normalize Avalon preservation bucket to use pairtrees

[davidschober]

Description

Avalon's preservation bucket currently stores files in a mix of pairtree and straight directory. We will normalize to pairtree

Done looks like

• preservation bucket stores all files in pairtree directories
• documentation written explaining storage scheme and "how to find a file"

[davidschober]

cc: @mbklein

[davidschober]

We will handle the move of all preservation assets to Meadow in the next year. This is moot. Closing.