Nodes with the same name can be written to non-SNS node

Refer to: https://community.jboss.org/thread/221483?tstart=0 for more background.

Essentially two threads trying to write to a node that doesn't allow SNS would succeed and SNS nodes would be created which results in a CVE when read (since it can't determine the proper type for the node anymore).

Test-case created in my github repository with the test showing the failure. It works properly when serialized but not so when running in parallel.

https://github.com/panghy/modeshape/commit/4d268ee8369a052fc329e72dc9c4ac33e258e211