Reword messages using "as" instead of "because"

Hello.

I noticed that the recent commit 0d48d393d46 introduced the following
three messages:

4793> errdetail("Retention is stopped as the apply process is not advancing its xmin within the configured max_retention_duration of %u ms.",
4822> ? errdetail("Retention is re-enabled as the apply process is advancing its xmin within the configured max_retention_duration of %u ms.",
4824> : errdetail("Retention is re-enabled as max_retention_duration is set to unlimited."));

I think I saw other instances of this kind of as recently, and I
thought we had agreed to avoid this usage and prefer because instead,
but I lost track of where that discussion took place.

Anyway, unlike some past uses, these ones are apparently confusing,
and I'd like to propose changing the wording to because.

In addition, I felt that the tense in the second message is not
immediately clear.  If it is reasonable and keeps the correct sense,
I'd like to propose changing "is (not) advancing its xmin within" to
"has (not) advanced its xmin into".

+ errdetail("Retention is stopped because the apply process has not advanced its xmin into the configured max_retention_duration of %u ms.",
+ ? errdetail("Retention is re-enabled because the apply process has advanced its xmin into the configured max_retention_duration of %u ms.",
+ : errdetail("Retention is re-enabled because max_retention_duration is set to unlimited."));

I'm not sure this is worth fixing, but anyway the proposed patch is
attached.

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center