Implement Request Retry and Backoff (#491)

Refactoring the timeout retries into a more general 'retry policy' with
 support for retrying on HTTP 429 (Too Many Requests) and other responses
 with a `Retry-After` header

The delay between requests is controlled by a combination of a 'backoff'
 (currently only a simple linear backoff), and honouring the
 `Retry-After` value (longest delay wins)

This makes the 'rate limiter' an optional argument as well, removing the
 use of `nil` when one isn't supplied

Signed-off-by: David Ackroyd <daveo.ackroyd@gmail.com>

engine/engine_types.go

@@ -59,7 +59,7 @@ type Settings struct {
 	EnableExchangeRESTSupport      bool
 	EnableExchangeWebsocketSupport bool
 	MaxHTTPRequestJobsLimit        int
-	RequestTimeoutRetryAttempts    int
+	RequestMaxRetryAttempts        int
 
 	// Global HTTP related settings
 	GlobalHTTPTimeout   time.Duration
@@ -67,9 +67,9 @@ type Settings struct {
 	GlobalHTTPProxy     string
 
 	// Exchange HTTP related settings
-	ExchangeHTTPTimeout   time.Duration
-	ExchangeHTTPUserAgent string
-	ExchangeHTTPProxy     string
+	HTTPTimeout   time.Duration
+	HTTPUserAgent string
+	HTTPProxy     string
 
 	// Dispatch system settings
 	EnableDispatcher        bool