Fix merge into

[osipovartem]

• to select correct columns we need to use selection alias a a prefix for column name
• added empty plan for Update statements and added check for references to include warehouse
  Fixes engine: Implement MERGE INTO statement #74

merge into datasets.public_snowplow_manifest.snowplow_web_base_quarantined_sessions trg
  using (select
      session_identifier

    from datasets.public_scratch.snowplow_web_base_sessions_this_run
    -- '=' since end_tstamp is restricted to start_tstamp + max_session_days
    where end_tstamp = 

    dateadd(
        day,
        3,
        start_tstamp
        )

) src
  on trg.session_identifier = src.session_identifier
  when not matched then insert (session_identifier) values(session_identifier);

this PR adding alias for fields in SELECT src.session_identifier ---->

INSERT INTO embucket.datasets.public_snowplow_manifest.snowplow_web_base_quarantined_sessions (session_identifier)
SELECT src.session_identifier
FROM (
    SELECT session_identifier
    FROM embucket.datasets.public_scratch.snowplow_web_base_sessions_this_run
    WHERE end_tstamp = dateadd(day, 3, start_tstamp)
) src
JOIN embucket.datasets.public_snowplow_manifest.snowplow_web_base_quarantined_sessions trg
ON trg.session_identifier = src.session_identifier
WHERE trg.session_identifier IS NULL;