[Cdash] buildfile cache and json tar issues

[Nils Gladitz]

While experimenting with gcov and java tar coverage submissions I 
noticed the following oddities:

- submitting gcov followed by java for the same build fails but it adds 
the java tar file to the buildfile cache nonetheless.

Submitting just the java file in a new build afterwards copies the gcov 
coverage from the previous build (even though gcov coverage was not part 
of the new submission).

- submitting java coverage with ctest_submit(CDASH_UPLOAD) creates rows 
in the coverage tables which a later ctest_submit() seems to remove again.

I think the buildfile cache still points to that build though so that 
later submissions for the same coverage data try to copy non existing 
table rows.

Removing the problematic entries from buildfile and submitting coverage 
data after everything else has been submitted seems to have fixed this 
somewhat.

Nils