MB-12546: Fix maximum node size calculation

When inserting a node, only the size of the existing node
was taken into account, without taking the newly added item
into account.

Change-Id: Ibc93ee1ea9bab292d74e8f63416fcf46c9564b80
Reviewed-on: http://review.couchbase.org/42832
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>