Add monotonic atomic counters for sequential ordering

[scullxbones]

• Use atomic findAndModify with counters per:
  • Global
  • Tag (when implemented)
• Migrate existing data to add global counter
• On append, grab batch via {$inc: N} and inject each into Atom
• Think about indexing

[filosganga]

@scullxbones I am not a MongoDB expert, my assumption was that a simple timestamp field on each Atom should be enough.

Even if the MongoDB documentation reads:

If you insert a document containing an empty BSON timestamp in a top-level field, the MongoDB server will replace that empty timestamp with the current timestamp value

I have no idea how you can insert an empty BSONTimestamp using Casbah or rxMongo.