Fixed by doing adding external_lock(F_UNLCK) in free_locks.
I also moved unlock of cache_handler to be after changing lock type of
cached tables to ensure that no one can use cached table while this is
happening (as cache table is locked with write lock).
I also fixed a wrong mutex order bug in ha_cache::flush_insert_cache()

Other things:
- Addded share::cached_rows to track inserted rows. This is only used
  for asserts to check the number of rows in the cache.
- Fixed wrong mysql_file_chsize() in case of repair