[server][ItemData] Don't initialize a TLS dynamically.

This patch is to suppress the following compile error on TravisCI (Ubuntu 12.04 LTS).

ItemData.cc: In member function ‘ItemGeneric<T, ITEM_TYPE>::operator const uint64_t&() const [with T = int, ItemDataType ITEM_TYPE = (ItemDataType)1u, uint64_t = long unsigned int]’:
ItemData.cc:187:33: error: ‘val’ is thread-local and so cannot be dynamically initialized
make[3]: *** [ItemData.lo] Error 1