don't issue requests that don't apply to query nodes, and if they do receive them, don't crash

[Dieterbe]

fix #1293
tested with docker-cluster-query

curl -v -s -i http://localhost:6061/metrics/delete -d "query=foo.bar.*"
*   Trying ::1...
* TCP_NODELAY set
* Connected to localhost (::1) port 6061 (#0)
> POST /metrics/delete HTTP/1.1
> Host: localhost:6061
> User-Agent: curl/7.64.1
> Accept: */*
> Content-Length: 15
> Content-Type: application/x-www-form-urlencoded
> 
* upload completely sent off: 15 out of 15 bytes
< HTTP/1.1 200 OK
HTTP/1.1 200 OK
< Content-Type: application/json
Content-Type: application/json
< Date: Thu, 25 Apr 2019 19:19:15 GMT
Date: Thu, 25 Apr 2019 19:19:15 GMT
< Server: Caddy
Server: Caddy
< Trace-Id: 77f0002c62c7ce6a
Trace-Id: 77f0002c62c7ce6a
< Vary: Accept-Encoding
Vary: Accept-Encoding
< Content-Length: 17
Content-Length: 17

< 
* Connection #0 to host localhost left intact
{"deletedDefs":0}* Closing connection 0

metrictank-q0_1  | [Macaron] 2019-04-25 19:19:15: Started POST /metrics/delete for 172.19.0.1
metrictank2_1    | [Macaron] 2019-04-25 19:19:15: Started POST /index/delete for 172.19.0.15
metrictank0_1    | [Macaron] 2019-04-25 19:19:15: Started POST /index/delete for 172.19.0.15
metrictank1_1    | [Macaron] 2019-04-25 19:19:15: Started POST /index/delete for 172.19.0.15
metrictank3_1    | [Macaron] 2019-04-25 19:19:15: Started POST /index/delete for 172.19.0.15
metrictank0_1    | [Macaron] 2019-04-25 19:19:15: Completed /index/delete 200 OK in 595.024µs
metrictank3_1    | [Macaron] 2019-04-25 19:19:15: Completed /index/delete 200 OK in 843.7µs
metrictank1_1    | [Macaron] 2019-04-25 19:19:15: Completed /index/delete 200 OK in 480.244µs
metrictank-q0_1  | [Macaron] 2019-04-25 19:19:15: Completed /metrics/delete 200 OK in 2.436699ms
metrictank2_1    | [Macaron] 2019-04-25 19:19:15: Completed /index/delete 200 OK in 753.214µs

curl -v -s -i http://localhost:6061/metrics/delete -d "query=some.id.of.a.metric.1*"
*   Trying ::1...
* TCP_NODELAY set
* Connected to localhost (::1) port 6061 (#0)
> POST /metrics/delete HTTP/1.1
> Host: localhost:6061
> User-Agent: curl/7.64.1
> Accept: */*
> Content-Length: 28
> Content-Type: application/x-www-form-urlencoded
> 
* upload completely sent off: 28 out of 28 bytes
< HTTP/1.1 200 OK
HTTP/1.1 200 OK
< Content-Type: application/json
Content-Type: application/json
< Date: Thu, 25 Apr 2019 19:20:18 GMT
Date: Thu, 25 Apr 2019 19:20:18 GMT
< Server: Caddy
Server: Caddy
< Trace-Id: 372447439ff27ee9
Trace-Id: 372447439ff27ee9
< Vary: Accept-Encoding
Vary: Accept-Encoding
< Content-Length: 19
Content-Length: 19

< 
* Connection #0 to host localhost left intact
{"deletedDefs":224}* Closing connection 0

metrictank-q0_1  | [Macaron] 2019-04-25 19:20:18: Started POST /metrics/delete for 172.19.0.1
metrictank3_1    | [Macaron] 2019-04-25 19:20:18: Started POST /index/delete for 172.19.0.15
metrictank0_1    | [Macaron] 2019-04-25 19:20:18: Started POST /index/delete for 172.19.0.15
metrictank2_1    | [Macaron] 2019-04-25 19:20:18: Started POST /index/delete for 172.19.0.15
metrictank1_1    | [Macaron] 2019-04-25 19:20:18: Started POST /index/delete for 172.19.0.15
metrictank1_1    | [Macaron] 2019-04-25 19:20:18: Completed /index/delete 200 OK in 36.641483ms
metrictank3_1    | [Macaron] 2019-04-25 19:20:18: Completed /index/delete 200 OK in 45.013004ms
metrictank2_1    | [Macaron] 2019-04-25 19:20:18: Completed /index/delete 200 OK in 47.188199ms
metrictank0_1    | [Macaron] 2019-04-25 19:20:18: Completed /index/delete 200 OK in 49.311407ms
metrictank-q0_1  | [Macaron] 2019-04-25 19:20:18: Completed /metrics/delete 200 OK in 51.236528ms

[robert-milan]

LGTM, aside from typo and broken unit tests.