MODE-1961 Fixed the processing of mixins when nodes are imported from an
XML file using IMPORT_UUID_COLLISION_REMOVE_EXISTING, multiple times. The problem was
caused by the fact that the same NodeKey is reused, causing in effect the
old persisted node(s) to be seen together with new nodes (which are
supposed to be created by the import). Thefore, mixins would not be set on
the new nodes because they appeared as "already existing".
MODE-1974 Corrected RepositoryStatistics

Made several corrections and improvements to the RepositoryStatistics
functionality, though most of the changes are for tests. Verified the
fix by running a test for 65 minutes (this test method is @Ingore'd
and will not run during any of the builds) and getting the same
windows as reported in the issue.
MODE-1961 Corrected compiler warnings from previous MODE-1961 commit