MongoDocumentStore: shrink in-DB documents after updates fail due to 16MB limit

To address the 16MB/childorder issue, there are many potential approaches:

• make GC more aggressive
• try to change updates to remove "in-between" changes of ":childOrder" property
• change the data model of ":childOrder"
• try to shrink document in DB once size related exception happens

This ticket is about the last of these options.

Proposal:

• improve exception thrown by document store so that it can be acted upon
• in document store utils add a method that inspects a document and produces UpdateOps suitable to shrink the document
• DocumentNodeStore commit could catch exception, obtain update ops, apply them, and retry once (this should be dependant on a feature toggle)