### Description

Currently async_search get or status response include the following info about async_search:

```
  "is_partial" : true, 
  "is_running" : true, 
  "start_time_in_millis" : 1583945890986,
  "expiration_time_in_millis" : 1584377890986,
```

In Kibana we would also like to have something like `completion_time` that would indicate time when the search was completed. It doesn't have to be precise, having some margin of error would be fine. 

**Some context:**

Currently Kibana uses background tasks to monitor completion of searches and it saves the completion time and then later shows it to users when they restore the search. Now we are simplifying the implementation of searches restoration on Kibana side and removing search monitoring in background tasks [(RFC here)](https://docs.google.com/document/d/1xpvW0xHmdm6w6EyYzYd01JLhiPFWF1Ij-tcJxCRLO7U/edit#) so Kibana can't longer track on its side the completion time. 

server/src/main/java/org/elasticsearch/TransportVersion.java
return new TransportVersion(id);
    }
    public static void writeVersion(TransportVersion version, StreamOutput out) throws IOException {
        out.writeVInt(version.id);
    }
    /**
     * Returns the minimum version of {@code version1} and {@code version2}
     */
    public static TransportVersion min(TransportVersion version1, TransportVersion version2) {
        return version1.id < version2.id ? version1 : version2;
    }
    /**
     * Returns the maximum version of {@code version1} and {@code version2}
     */
    public static TransportVersion max(TransportVersion version1, TransportVersion version2) {
        return version1.id > version2.id ? version1 : version2;
    }
    /**
     * Returns {@code true} if the specified version is compatible with this running version of Elasticsearch.
     */
    public static boolean isCompatible(TransportVersion version) {
        return version.onOrAfter(MINIMUM_COMPATIBLE);
    }
    /**
     * Reference to the most recent transport version.
     * This should be the transport version with the highest id.
     */
    public static TransportVersion current() {
        return CurrentHolder.CURRENT;