plugin/rewrite: pre-compile CNMAE rewrite regexp (#7697)

This commit changes the CNAME rewrite rule to use a pre-compiled regexp
when the match type is RegexMatch instead of compiling it on-the-fly for
each request. This will also allow for invalid regexp patterns to be
identified during setup instead of causing a panic when the rule is
first invoked.

Signed-off-by: Charlie Vieth <charlie.vieth@gmail.com>

plugin/rewrite/setup_test.go

@@ -33,6 +33,7 @@ func TestParse(t *testing.T) {
 }`, true, "must begin with a name rule"},
 		{`rewrite stop`, true, ""},
 		{`rewrite continue`, true, ""},
+		{`rewrite stop name regex [bad[ bar answer name bar foo`, true, ""},
 	}
 
 	for i, test := range tests {