Resolve consistency expectations of the OMNIBOR_DIR

[alilleybrinker]

@bharsesh raised multiple questions in #39 relating to any consistency expectations for the OMNIBOR_DIR, including:

• Can the OMNIBOR_DIR be assumed to be write-only?
• Can OmniBOR documents be read during writes to the OMNIBOR_DIR?
• Do we want to require that any writer to OMNIBOR_DIR can read their own writes?
• Is a tool allowed to read and write within the OMNIBOR_DIR at the same time?
• How do we handle two tools attempting to read/write within the OMNIBOR_DIR at the same time?

This basically comes down to what sort of locking behavior, if any, we want to require. Considerations almost certainly include that many build tools already implement their own locking behavior for build directories and provide associated guarantees for artifacts. This also gets into the vagaries of commitment of data by different filesystems, so there may be limits on what we can ourselves require / guarantee.