Added key buffer local to each StorageCursor
* Reduced memory allocs
* Increased steps-before-jump of LogicAndIterator to 10
Made BufferGuard size type dependent
* Leads to type safety of BufferGuard operations
Fixed memory leak in LevelDB option (StorageGuard)
* Changed templates based on Storage-Key classes to 'class' instead of 'typename'
Edge- and PropertyIterator implementation cleanup