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