Bug fix for the concurrent create/drop and list of namespaces.

See #867 (NSS concurrency problem with list namespaces and create namespace)


git-svn-id: https://svn.code.sf.net/p/bigdata/code/branches/BIGDATA_RELEASE_1_3_0@8016 8f7bc0f5-282e-0410-95e3-8d296e9bb460
minor changes (javadoc edit, private method, final attribute).

git-svn-id: https://svn.code.sf.net/p/bigdata/code/branches/BIGDATA_RELEASE_1_3_0@8019 8f7bc0f5-282e-0410-95e3-8d296e9bb460
Added test suite for #868 (COUNT DISTINCT does not return any solutions if there are no solutions in the data that flow into that operator)

git-svn-id: https://svn.code.sf.net/p/bigdata/code/branches/BIGDATA_RELEASE_1_3_0@8020 8f7bc0f5-282e-0410-95e3-8d296e9bb460