Delete expired garbage even when there is no GC pack present.

Delete expired garbage even when there is no GC pack present.

Delete the condition to check whether the garbage pack creation time
is older than the last GC operation, because it's not possible to
find the last GC operation time when there is no GC pack.

Add additional tests to make sure the contents of the expired garbage
packs are considered during the GC operation and any actively
referenced objects from the garbage packs are copied successfully
into the GC pack before deleting the garbage pack.

Change-Id: I09e8b2656de8ba7f9b996724ad1961d908e937b6
Signed-off-by: Thirumala Reddy Mutchukota <thirumala@google.com>