Must set all required fields before set builder on parent deltas.
Added test to serialize a single threaded map variable.  Still need to
add test to deserialize it.  Similarly, need to add code that tests
changing just individual values in the map.
Added functionality in var store to retrieve container objects from
other side and incorporate their deltas/changes on partner.  Still
unsure if it will really work.  Will fill in incorporate_deltas on
maps to see if will work.
Added code for deserializing changes in map.
Apparently protocol buffers clones builders when you add them.  This
means that the builders that you add will point at different data than
the builder you used to add.  Ie, you don't maintain a reference to
the parent's builder, instead you have a dumb empty copy.  This change
ensures then that I only add builders to parents *after* I add all
builders to them.