file locking and semaphore issues

Many things should be fixed or improved about file locking and semaphores:

• use different methods to remove files or directories
• replace the buggy semaphore usage (IPC trigger) by file / directory last changes and generation numbers (I suggest an unsigned long at the beginning of object files)
• use flock in place of fcntl lock when available
• store() before commitTransaction (cf the comment...)
• use a different file for transaction locking (I suggest <uuid>.lock in parallel to the current <uuid>.object) and of course use an exclusive (aka write) lock...
  BTW try to fix the multithreading issue...