Avoid unnecessary copy value and map.find in lru_cache

[cheerconi]

The operator[] is a find-or-add operator. Use m_map.emplace to avoid add operation. And use i->second.second to avoid find operation.

[coveralls]

Coverage decreased (-0.006%) to 84.013% when pulling d0e7f5a on cheerconi:master into 36c8913 on boostorg:master.