MVRBTreeRID: fixed bugs on removing last persistent items

Before this patch it removed also all the new items because it called
clear(). Now tests about the size of the tree. Fixed also other parts
where the size() was asked but the real tree size is needed.
Configuration (mvrbtree.ridNodePageSize): enlarged node page size from 16 to 64 items for MVRBTree RID sets
Fixed bug on reusing of iterators
Merge branch 'master' of https://github.com/nuvolabase/orientdb
Improved LazyWrapper to avoid copy collection in memory