memcache.Memcache.mget should use const

[everwanna]

diff:

307c307
< bool mget(std::vector<std::string> &keys)
---
> bool mget(const std::vector<std::string> &keys)
320c320
< std::vector<std::string>::iterator it= keys.begin();
---
> std::vector<std::string>::const_iterator it= keys.begin();