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.
Add support for sparse entries in FileDB.
DO NOT MERGE.