Performance Enhancements

- Nibbler: Now locally stores input length rather than repeatedly calling
  std::string::length.
- Nibbler: Makes greater use of std::string::find_first_not_of, instead of
  looping.
- TDB: No longer applies empty filters to lists - just copies the lists.
- TDB: Now caches data when reading completed.data.
- TDB: During loadPending and loadCompleted, makes fewer copies of the data.
- TDB: In commit, breaks out of search loops after finding the right data.
- TDB: In gc, only writes out minimal pending or completed data, instead of
       all data, all the time.
- TDB: No longer reads completed.data in gc, and simply appends completed
       and deleted tasks to it.

src/Nibbler.h

@@ -55,6 +55,7 @@ public:
 
 private:
   std::string mInput;
+  std::string::size_type mLength;
   std::string::size_type mCursor;
 };