nsqd: faster priority queue

Just a branch I had laying around...

Will only affect consumption side, benchmarks:

**old**
```
PUB: 2014/06/20 09:41:31 duration: 1.27351877s - 149.770mb/s - 785225.961ops/s - 1.274us/op
SUB: 2014/06/20 09:41:37 duration: 5.346942005s - 35.672mb/s - 187022.788ops/s - 5.347us/op
```

**new**
```
PUB: 2014/06/20 09:41:55 duration: 1.27610215s - 149.467mb/s - 783636.326ops/s - 1.276us/op
SUB: 2014/06/20 09:42:00 duration: 5.051318506s - 37.759mb/s - 197968.114ops/s - 5.051us/op
```