Make it easier to reuse DocumentMetadataHandle

[jmakeig]

I'd like to create a default metadata strategy and then extend that with custom properties each time I use it, for example, to add a batch ID collection when ingesting a batch. The with* methods on DocumentMetadataHandle, change the internal state and return this. Instead, it would be nice to modify state on a clone and return that.

final DocumentMetadataHandle meta  = new DocumentMetadataHandle() //
                .withCollections("raw", "sakila", "jdbc:mysql://localhost:3306/sakila") //
                .withPermission("bigpicture-reader", Capability.READ) //
                .withPermission("bigpicture-writer", Capability.INSERT, Capability.UPDATE);
…

// First batch
doc.write(…, meta.withCollections("batch-1234"));

…
// Second batch
doc.write(…, meta.withCollections("batch-5678"));

Instead, since the state of meta is changed with the withCollections(), the second invocation has both collections, not just the base plus the second batch collection.

[sammefford]

Related to #646