Proposal: add a debug message about using geqo

Hi,

During query tuning, users may want to check if GEQO is used or not
to generate a plan. However, users can not know it by simply counting
the number of tables that appear in SQL. I know we can know it by
enabling GEQO_DEBUG flag, but it needs recompiling, so I think it is
inconvenient. 

So, I would like to propose to add a debug level message that shows
when PostgreSQL use GEQO. That enables users to easily see it by
just changing log_min_messages.

Use cases are as follows:
- When investigating about the result of planning, user can determine
whether the plan is chosen by the standard planning or GEQO.

- When tuning PostgreSQL, user can determine the suitable value of
geqo_threshold parameter.

Best regards.

-- 
KAWAMOTO Masaya 
SRA OSS, Inc. Japan