VanillaChronicle fixes

- Fix for concurrent append in VanillaIndexCache
  Concurrent access to the append method was causing
  a buffer overflow failure because the position was being
  incremented by another thread after the check for end of file.

- Fix for VanillaIndexCache.lastIndexFile
  lastIndexFile method was not listing existing index files

- Fix for VanillaDataCacheTest.testDataForLast
  VanillaDataCache was not listing existing files

- Unit tests for VanillaTailer reading across cycles