Support using redis urls to construct the redis client (#1994)

Currently Athens only supports connecting to Redis using a hostname:port combination in addition to a password. While this works in most cases it also means that if you have other options you wish to supply Athens has to be updated to support them. As a basic example Redis clusters that require TLS currently are not supported by Athens but with this change you can simply supply a [redis url](https://github.com/redis/redis-specifications/blob/master/uri/redis.txt) to connect over TLS. It also makes it easy to override the password, set a username and more all from a single configuration option:

`rediss://username:password@redis.example.com:6379/1?protocol=3`

docs/content/configuration/storage.md

@@ -445,6 +445,22 @@ You can also optionally specify a password to connect to the redis server with
             # Env override: ATHENS_REDIS_PASSWORD
             Password = ""
 
+Connecting to Redis via a [redis url](https://github.com/redis/redis-specifications/blob/master/uri/redis.txt) is also
+supported:
+
+    SingleFlightType = "redis"
+
+    [SingleFlight]
+        [SingleFlight.Redis]
+            # Endpoint is the redis endpoint for the single flight mechanism
+            # Env override: ATHENS_REDIS_ENDPOINT
+            # Note, if TLS is required use rediss:// instead.
+            Endpoint = "redis://user:password@127.0.0.1:6379:6379/0?protocol=3"
+
+If the redis url is invalid or cannot be parsed, Athens will fall back to treating `Endpoint` as if it were
+a normal `host:port` pair. If a password is supplied in the redis url, in addition to being provided in the `Password`
+configuration option, the two values must match otherwise Athens will fail to start.
+
 ##### Customizing lock configurations:
 If you would like to customize the distributed lock options then you can optionally override the default lock config to better suit your use-case: