feat(drainer): added drainer which reads from redis stream and executes queries on DB (#142)

crates/router/src/services.rs

@@ -41,7 +41,7 @@ impl Store {
 
     #[cfg(feature = "kv_store")]
     pub fn drainer_stream(&self, shard_key: &str) -> String {
-        // "{shard_key}_stream_name"
+        // Example: {shard_5}_drainer_stream
         format!("{{{}}}_{}", shard_key, self.config.drainer_stream_name,)
     }
 }