Better locality for blobs collections over sharding

Currently when use oak with mongo mk for blobs storage we can easily end up with all different chunks of a binary streams scattered across the shards.

Now this is not ideal since it generates a large number of scattered gather queries over the shards for each individual files.

To allow better locality I propose the addition of another field called _anchor
This anchor field will be generated by the inverse order of the current timestamp of beginning of the storage of the file:

//Milliseconds Second Minute HH
SimpleDateFormat sdf = new SimpleDateFormat("SSSssmmHH");
//store the parsed integer of this value for more storage efficiency
String a = asdf.format(new Date());
int _anchor = Integer.parseInt(asdf.format(new Date()));

This new _anchor field should be part of the shard key which also requires to be indexed along side with _id

Pull request is on the making!

N.