Retention config shouldn't need logstream to be initialized (#398)

It is common for users to create stream and set the retention
(and be done with initial config) before sending logs to the
log stream.

Accordingly we're going to add support in Helm chart to
declaratively create logstream and set retention.

To allow that setup we need to ensure log stream retention
can be set before log stream has events (i.e. initialised)

Prerequisite for #397

Author: nitisht

server/src/handlers/http/logstream.rs

@@ -212,10 +212,6 @@ pub async fn put_retention(
         Err(err) => return Err(StreamError::InvalidRetentionConfig(err)),
     };
 
-    if !STREAM_INFO.stream_initialized(&stream_name)? {
-        return Err(StreamError::UninitializedLogstream);
-    }
-
     CONFIG
         .storage()
         .get_object_store()