RFC: Logging plan of the running query

Hi,

I liked this idea and after reviewing code I noticed some moments and 
I'd rather ask you some questions.


Firstly, I suggest some editing in the comment of commit. I think, it is 
turned out the more laconic and the same clear. I wrote it below since I 
can't think of any other way to add it.

```
Currently, we have to wait for finishing of the query execution to check 
its plan.
This is not so convenient in investigation long-running queries on 
production
environments where we cannot use debuggers.

To improve this situation there is proposed the patch containing the 
pg_log_query_plan()
function which request to log plan of the specified backend process.

By default, only superusers are allowed to request log of the plan 
otherwise
allowing any users to issue this request could create cause lots of log 
messages
and it can lead to denial of service.

At the next requesting CHECK_FOR_INTERRUPTS(), the target backend logs 
its plan at
LOG_SERVER_ONLY level and therefore this plan will appear in the server 
log only,
not to be sent to the client.
```

Secondly, I have question about deleting USE_ASSERT_CHECKING in lock.h?
It supposed to have been checked in another placed of the code by 
matching values. I am worry about skipping errors due to untesting with 
assert option in the places where it (GetLockMethodLocalHash) 
participates and we won't able to get core file in segfault cases. I 
might not understand something, then can you please explain to me?

Thirdly, I have incomprehension of the point why save_ActiveQueryDesc is 
declared in the pquery.h? I am seemed to save_ActiveQueryDesc to be used 
in an once time in the ExecutorRun function and  its declaration 
superfluous. I added it in the attached patch.

Fourthly, it seems to me there are not enough explanatory comments in 
the code. I also added them in the attached patch.

Lastly, I have incomprehension about handling signals since have been 
unused it before. Could another signal disabled calling this signal to 
log query plan? I noticed this signal to be checked the latest in 
procsignal_sigusr1_handler function.

Regards,

--
Alena Rybakina
Postgres Professional