Cache system bug fixes and performance improvement

Changed

• Improved the performance of directory listing generation while there are
  on-going file transfers
• Wrapped mutex locking and unlocking functions in error checking functions.

Fixed

• Fixed issue #40 - Crashes with "API function called from within callback".
• Cache system: now keep track of the number of times a cache file has been
  opened.
  • The on-disk cache file no longer gets opened multiple times, if
    a file is opened multiple times. This used to cause inconsistencies
    between two opened cache files.
• Cache system: Fixed buffer over-read at the boundary.
  • Say we are using a lock size of 1024k, we send a request for 128k at
    1008k. It won't trigger the download, because we have already downloaded the
    first 1024k at byte 0. So it would read off from the empty disk space!
  • This problem only occurred during the first time you download a file.
    During subsequent accesses, when you are only reading from the cache, this
    problem did not occur.
• Cache system: Previously it was possible for Cache_bgdl()'s download offset
  to be modified by the parent thread after the child thread had been
  launched. This used to cause permanent cache file corruption.
• Cache system: Cache_bgdl() no longer prefetches beyond EOF.
• Cache system: Data_read() no longer gives warning messages when reaching the
  end of the cache file.