nsqd: http api inconsistencies

It's a little weird to use GETs, but the docs don't mention which verb to use. It would be nicer to have `DELETE host/topic/test` or the POST at least just for consistency, POSTS are definitely more curl-friendly so maybe going full "rest" (meh) is not worth it

```
curl -d topic=test http://127.0.0.1:4151/delete_topic
{"status_code":500,"status_txt":"MISSING_ARG_TOPIC","data":null}
```

```
curl http://127.0.0.1:4151/delete_topic?topic=test
{"status_code":200,"status_txt":"OK","data":null}
```