Refactor: prepare CatalogTraversal for timestamp pruning
add timestamp pruning to CatalogTraversal
TEST: annotate MockCatalog with modification timestamp
TEST: unit test for timestamp bases pruning
traversal document update
Refactor: fully centralize CloseCatalog()
FIX: GC unit tests for time threshold
FIX: missing $CVMFS_TEST_REPO in test 560
Merge pull request #611 from reneme/fix-test_560

Fix: Test 560 did not Properly Use $CVMFS_TEST_REPO