HTTP requests on in-flight queries blocks query execution in coordinator side

This is a follow-up task for IMPALA-13034.

HTTP requests on in-flight queries usually acquire the lock of ClientRequestState. This could blocks client requests in fetching query results. E.g. there are several endpoints in WebUI that can dump a query profile: /query_profile, /query_profile_encoded, /query_profile_plain_text, /query_profile_json. If the profile is huge, such requests impact the query performance.

Fetching the details (profile, exec summary, etc.) of an in-flight query has lower priority and shouldn't block query execution.