plugin/route53: make refresh frequency adjustable (#3083)

the current update frequency for the refresh loop in the route 53 plugin is hard-coded
to 1 minute. aws rate-limits the number of api requests so less frequent record refreshes
can help when reaching those limits depending upon your individual scenarios. this pull
adds a configuration option to the route53 plugin to adjust the refresh frequency.

thanks for getting my last pull released so quickly. this is the last local change that
i have been running and would love to get it contributed back to the project.

Signed-off-by: Matt Kulka <mkulka@parchment.com>

plugin/route53/README.md

@@ -18,6 +18,7 @@ route53 [ZONE:HOSTED_ZONE_ID...] {
     aws_access_key [AWS_ACCESS_KEY_ID AWS_SECRET_ACCESS_KEY]
     credentials PROFILE [FILENAME]
     fallthrough [ZONES...]
+    refresh DURATION
 }
 ~~~
 
@@ -48,6 +49,14 @@ route53 [ZONE:HOSTED_ZONE_ID...] {
 *   **ZONES** zones it should be authoritative for. If empty, the zones from the configuration
     block.
 
+*   `refresh` can be used to control how long between record retrievals from Route 53. It requires
+    a duration string as a parameter to specify the duration between update cycles. Each update
+    cycle may result in many AWS API calls depending on how many domains use this plugin and how
+    many records are in each. Adjusting the update frequency may help reduce the potential of API
+    rate-limiting imposed by AWS.
+
+*   **DURATION** A duration string. Defaults to `1m`. If units are unspecified, seconds are assumed.
+
 ## Examples
 
 Enable route53 with implicit AWS credentials and resolve CNAMEs via 10.0.0.1:
@@ -86,3 +95,12 @@ Enable route53 with multiple hosted zones with the same domain:
     route53 example.org.:Z1Z2Z3Z4DZ5Z6Z7 example.org.:Z93A52145678156
 }
 ~~~
+
+Enable route53 and refresh records every 3 minutes
+~~~ txt
+. {
+    route53 example.org.:Z1Z2Z3Z4DZ5Z6Z7 {
+      refresh 3m
+    }
+}
+~~~