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/cache/freq/freq.go

@@ -0,0 +1,55 @@
+// Package freq keeps track of last X seen events. The events themselves are not stored
+// here. So the Freq type should be added next to the thing it is tracking.
+package freq
+
+import (
+	"sync"
+	"time"
+)
+
+// Freq tracks the frequencies of things.
+type Freq struct {
+	// Last time we saw a query for this element.
+	last time.Time
+	// Number of this in the last time slice.
+	hits int
+
+	sync.RWMutex
+}
+
+// New returns a new initialized Freq.
+func New(t time.Time) *Freq {
+	return &Freq{last: t, hits: 0}
+}
+
+// Update updates the number of hits. Last time seen will be set to now.
+// If the last time we've seen this entity is within now - d, we increment hits, otherwise
+// we reset hits to 1. It returns the number of hits.
+func (f *Freq) Update(d time.Duration, now time.Time) int {
+	earliest := now.Add(-1 * d)
+	f.Lock()
+	defer f.Unlock()
+	if f.last.Before(earliest) {
+		f.last = now
+		f.hits = 1
+		return f.hits
+	}
+	f.last = now
+	f.hits++
+	return f.hits
+}
+
+// Hits returns the number of hits that we have seen, according to the updates we have done to f.
+func (f *Freq) Hits() int {
+	f.RLock()
+	defer f.RUnlock()
+	return f.hits
+}
+
+// Reset resets f to time t and hits to hits.
+func (f *Freq) Reset(t time.Time, hits int) {
+	f.Lock()
+	defer f.Unlock()
+	f.last = t
+	f.hits = hits
+}