[PATCH] Fix CertficateDB locking scheme

Fix CertficateDB locking scheme

Currently we are locking every file going to be accessed by
CertificateDB code even if it is not realy needed, because of a more
general lock.

This patch:
    - Replace the old FileLocker class with the pair Lock/Locker classes
    - Remove most of the locks in CertificateDB with only two locks one
      for main database locking and one lock for the file contain the
      current serial number.

This is a Measurement Factory project