Add Client Manager to monitor connection state and trigger reconnect (#39)

- Support for exponential backoff on reconnect to be gentle on the server.
- Clean up client by moving metrics and retry strategy to the connection manager.
- Update echo_client to use client manager
- Fix echo client XMPP message matching

Fixes #21
Improvements for #8

backoff.go

@@ -0,0 +1,101 @@
+/*
+Interesting reference on backoff:
+- Exponential Backoff And Jitter (AWS Blog):
+  https://www.awsarchitectureblog.com/2015/03/backoff.html
+
+We use Jitter as a default for exponential backoff, as the goal of
+this module is not to provide precise 'ticks', but good behaviour to
+implement retries that are helping the server to recover faster in
+case of congestion.
+
+It can be used in several ways:
+- Using duration to get next sleep time.
+- Using ticker channel to trigger callback function on tick
+
+The functions for Backoff are not threadsafe, but you can:
+- Keep the attempt counter on your end and use DurationForAttempt(int)
+- Use lock in your own code to protect the Backoff structure.
+
+TODO: Implement Backoff Ticker channel
+TODO: Implement throttler interface. Throttler could be used to implement various reconnect strategies.
+*/
+
+package xmpp // import "gosrc.io/xmpp"
+
+import (
+	"math"
+	"math/rand"
+	"time"
+)
+
+const (
+	defaultBase   int = 20 // Backoff base, in ms
+	defaultFactor int = 2
+	defaultCap    int = 180000 // 3 minutes
+)
+
+// Backoff can provide increasing duration with the number of attempt
+// performed. The structure is used to support exponential backoff on
+// connection attempts to avoid hammering the server we are connecting
+// to.
+type Backoff struct {
+	NoJitter     bool
+	Base         int
+	Factor       int
+	Cap          int
+	lastDuration int
+	attempt      int
+}
+
+// Duration returns the duration to apply to the current attempt.
+func (b *Backoff) Duration() time.Duration {
+	d := b.DurationForAttempt(b.attempt)
+	b.attempt++
+	return d
+}
+
+// Wait sleeps for backoff duration for current attempt.
+func (b *Backoff) Wait() {
+	time.Sleep(b.Duration())
+}
+
+// DurationForAttempt returns a duration for an attempt number, in a stateless way.
+func (b *Backoff) DurationForAttempt(attempt int) time.Duration {
+	b.setDefault()
+	expBackoff := math.Min(float64(b.Cap), float64(b.Base)*math.Pow(float64(b.Factor), float64(b.attempt)))
+	d := int(math.Trunc(expBackoff))
+	if !b.NoJitter {
+		d = rand.Intn(d)
+	}
+	return time.Duration(d) * time.Millisecond
+}
+
+// Reset sets back the number of attempts to 0. This is to be called after a successfull operation has been performed,
+// to reset the exponential backoff interval.
+func (b *Backoff) Reset() {
+	b.attempt = 0
+}
+
+func (b *Backoff) setDefault() {
+	if b.Base == 0 {
+		b.Base = defaultBase
+	}
+
+	if b.Cap == 0 {
+		b.Cap = defaultCap
+	}
+
+	if b.Factor == 0 {
+		b.Factor = defaultFactor
+	}
+}
+
+/*
+We use full jitter as default for now as it seems to provide good behaviour for reconnect.
+
+Base is the default interval between attempts (if backoff Factor was equal to 1)
+
+Attempt is the number of retry for operation. If we start attempt at 0, first sleep equals base.
+
+Cap is the maximum sleep time duration we tolerate between attempts
+*/