Re: Introduce XID age and inactive timeout based replication slot invalidation

Please find the updated v78 patches after a few off-list review rounds.

Here is a summary of changes in v78:
patch-001:
- Fixed bugs reported by Hou-san and Peter in [1] and [2].
- Fixed a race condition reported by Hou-san off-list, which could
lead to an assert failure.
This failure happens when the checkpointer sets the invalidation cause
to idle_timeout on the first attempt, but if it later finds another
process's pid active for the slot, it retries after terminating that
process. By then, inactive_since may have been updated, and it
determines the invalidation_cause as RS_INVAL_NONE and below assert
fails:

```
Assert(!(invalidation_cause_prev != RS_INVAL_NONE && terminated &&
invalidation_cause_prev != invalidation_cause));
```

- Moved the slot's idle_time calculation to the caller of
ReportSlotInvalidation().
- Improved the patch commit message for better clarity.

patch-002:
- Fixed a bug reported by Kuroda-san - "check_extension() must be done
before the CREATE EXTENSION".
- Addressed a few other comments by Peter and Kuroda-san to optimize
code and improve comments.

[1] https://www.postgresql.org/message-id/CABdArM7eeejXEgd6t4wtBiK%3DaWc%2B%2Bgt1__WwAWm-Y_5xMVskWg%40mail.gmail.com
[2] https://www.postgresql.org/message-id/CAHut%2BPtnWyOMvxb6mZHWFxqD-NdHuYL8Zp%3D-QasAQ3VvxauiMA%40mail.gmail.com

--
Thanks,
Nisha