Size based BlockDiskCache data corruption

As a follow-up to JCS-147 - I've encountered some problems when using BlockDiskCache and size limitation that sometimes content is dropped due to failed integrity check. It's not reproducible every time, but I've managed to create the JUnit test that fails every time just by using simple loop.

From attached patch, testLoadFromDisk fails on my system every time. I've tried to debug this, but I've run out of ideas, where to look for the problem.

Attached patch contains also some restructure of the tests:
BlockDiskCacheUnitTest renamed to BlockDiskCacheUnitTestAbstract
IndexDiskCacheUnitTest renamed to IndexDiskCacheUnitTestAbstract
IndexDiskCacheUnitTestCount ->IndexDiskCacheCountUnitTest
IndexDiskCacheUnitTestSize -> IndexDiskCacheSizeUnitTest
BlockDiskCacheUnitTestCount -> BlockDiskCacheCountUnitTest
BlockDiskCacheUnitTestSize -> BlockDiskCacheSizeUnitTest

To better align with current patterns looking for tests.