Add support for async serializer so it can be used with worker threads

index.ts

@@ -12,7 +12,7 @@ interface NormalizedRedisClient {
 }
 
 interface Serializer {
-  parse(s: string): SessionData
+  parse(s: string): SessionData | Promise<SessionData>
   stringify(s: SessionData): string
 }
 
@@ -83,7 +83,7 @@ class RedisStore extends Store {
     try {
       let data = await this.client.get(key)
       if (!data) return cb()
-      return cb(null, this.serializer.parse(data))
+      return cb(null, await this.serializer.parse(data))
     } catch (err) {
       return cb(err)
     }

readme.md

@@ -113,9 +113,11 @@ This option disables key expiration requiring the user to manually manage key cl
 
 Provide a custom encoder/decoder to use when storing and retrieving session data from Redis (default: `JSON.parse` and `JSON.stringify`).
 
+Optionally `parse` method can be async if need be.
+
 ```ts
 interface Serializer {
-  parse(string): object
+  parse(string): object | Promise<object>
   stringify(object): string
 }
 ```