Allow ThreadPool to be configured with an arbitrary number of threads.
Do not hold the lock more than is necessary in FileDB.
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.
Add support for sparse entries in FileDB.
DO NOT MERGE.