Add a libevent::Base::Run() method.

This is similar to ThreadPool::Add(), but runs the closure through libevent,
ensuring serialization with other libevent callbacks.
Read the latest STH even if there are no entries in FileDB.
Make SQLiteDB<>::db_ const.

For my own peace of mind...
Make the SQLiteDB<> class threadsafe.
Make a few pointer members of FileDB<> const.

Also check them for null.
Make the FileDB<> class threadsafe.