global: move to context (#1699)

* global: move to context

Move from golang.org/x/net/context to std lib's context.

Change done with:

for i in $(grep -l '/context' **/*.go); do sed -e 's|golang.org/x/net/context|context|' -i $i; echo $i; done
for i in **/*.go; do goimports -w $i; done

* drop from dns.pb.go as well

plugin/log/setup_test.go

@@ -94,16 +94,13 @@ func TestLogParse(t *testing.T) {
 		}}},
 		{`log {
 			class abracadabra
-		}`, true, []Rule{
-		}},
+		}`, true, []Rule{}},
 		{`log {
 			class
-		}`, true, []Rule{
-		}},
+		}`, true, []Rule{}},
 		{`log {
 			unknown
-		}`, true, []Rule{
-		}},
+		}`, true, []Rule{}},
 	}
 	for i, test := range tests {
 		c := caddy.NewTestController("dns", test.inputLogRules)