Use google logging instead of assert.h.
Use "int" instead of "unsigned int" (as per the style guide).
A few changes to improve robustness of FileStorage.

Also, my Seal of Approval (tm) for thread safety. ;-)
Make the header guard conform to the style guide.
Use std::make_pair, rather than the std::pair contructor.
Use google logging instead of assert.h.
Small improvement in how FileDB lookups return entries.
Put the members of FileDB protected by lock_ together.
Add more verification of the hash of entries.
Do not hold the lock more than is necessary in FileDB.