Remove the word middleware (#1067)

* Rename middleware to plugin

first pass; mostly used 'sed', few spots where I manually changed
text.

This still builds a coredns binary.

* fmt error

* Rename AddMiddleware to AddPlugin

* Readd AddMiddleware to remain backwards compat

plugin/dnssec/cache.go

@@ -0,0 +1,22 @@
+package dnssec
+
+import (
+	"hash/fnv"
+
+	"github.com/miekg/dns"
+)
+
+// hash serializes the RRset and return a signature cache key.
+func hash(rrs []dns.RR) uint32 {
+	h := fnv.New32()
+	buf := make([]byte, 256)
+	for _, r := range rrs {
+		off, err := dns.PackRR(r, buf, 0, nil, false)
+		if err == nil {
+			h.Write(buf[:off])
+		}
+	}
+
+	i := h.Sum32()
+	return i
+}