replace map count/insert w/emplace in instantx.cpp (#2165)
replace map count/insert w/emplace in instantx.cpp (#2165)

* replace map count/insert w/emplace in instantx.cpp

This feels like an optimization, something about:

```
if map.count(key)
     return false
else
     insert(pair)
     return true
```

... just feels icky. Plus, `vote.GetMasternodeOutpoint()` is only called
once. The previous version might be slightly more readable, however.

* use std::pair template constructor shortcut


Former-commit-id: 34153ccf79f41cb7856ff7e2065db6e3171d845c
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin