[filebrowser] fix some potential bugs in LRUCache and datamgr

fix a bug according to coverity report
    5. deref_arg: Calling contains dereferences freed pointer path_cache.

fix a bug that method LRUCache:;contain returns true while LRUCache::operator[]
returns NULL
    now operator[] won't remove any item

now instantiate template class LRUCache properly and don't need to include the
header of LRUCache

remove some extra semis