cache promote plugin takes too much calculating size of LRUList

[vmamidi]

pstack shows that cache promote plugin spends lot of time calculating too much time in determining the size of LRUList

#0 std::__distance<std::_List_const_iterator<std::pair<LRUHash, unsigned int> > > (__last=..., __first=...) at /opt/gcc/include/c++/4.9.3/bits/stl_iterator_base_funcs.h:83
#1 std::distance<std::_List_const_iterator<std::pair<LRUHash, unsigned int> > > (__last=..., __first=...) at /opt/gcc/include/c++/4.9.3/bits/stl_iterator_base_funcs.h:118
#2 std::list<std::pair<LRUHash, unsigned int>, std::allocator<std::pair<LRUHash, unsigned int> > >::size (this=0x2b37fe095c58) at /opt/gcc/include/c++/4.9.3/bits/stl_list.h:887

using a length variable should make it not dependent on stl list implementation