RE: [Proposal] Add foreign-server health checks infrastructure

Dear Horiguchi-san, Fujii-san,

> > I understood here as removing following mechanism from core:
> >
> > * disable timeout at end of tx.
> > * skip if held off or read commands
>
> I think we're on the same page. Anyway query cancel interrupt is
> ignored while rading input.
>
> > > - If an existing connection is found to be dead, just try canceling
> > > the query (or sending query cancel).
> > > One issue with it is how to show the decent message for the query
> > > cancel, but maybe we can have a global variable that suggests the
> > > reason for the cancel.
> >
> > Currently I have no good idea for that but I'll try.
>
> However, I would like to hear others' opnions about the direction, of
> course.
>

Based on the idea, I re-implemented the feature. Almost all feature is
moved to postgres_fdw. The abstract of my patch is as follows:

# core

* Exposes QueryCancelMessage for error reporting
* Uses above if it was not NULL

# postgres_fdw

* Defines new GUC postgres_fdw.health_check_interval.
It is renamed simpler.
* Registers a timeout when initializing connection hash.
* Raises SIGINT and sets QueryCancelMessage to message.
if detects a connection lost.

I also attached a test as zipped file for keep cfbot quiet.
When connection lost is detected, the following message will show:

```
ERROR: Foreign Server (servername) might be down.
```

How do you think?

Best Regards,
Hayato Kuroda
FUJITSU LIMITED