Disable vacuum's use of a buffer access strategy during failsafe

Traditionally, vacuum always makes use of a buffer access strategy 32
buffers in size.  This means that running vacuums tend not to cause too
many shared buffers to become dirty, however, this can cause vacuums to
run much more slowly than they otherwise could as WAL flushes will occur
more frequently due to having to flush WAL out to the LSN of the dirty
page before that page can be written to disk.

When we are performing failsafe VACUUMs (as added in 1e55e7d17), we really
want to make the vacuum work go as quickly as possible, so here we disable
the buffer access strategy when entering failsafe mode while vacuuming a
relation.

Per idea and analyis from Andres Freund.

In passing, also include some changes I had intended for 32fbe0239.

Author: Melanie Plageman
Reviewed-by: Justin Pryzby, David Rowley
Discussion: https://postgr.es/m/20230111182720.ejifsclfwymw2reb%40awork3.anarazel.de

doc/src/sgml/config.sgml

@@ -9320,8 +9320,12 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
        </para>
        <para>
         When the failsafe is triggered, any cost-based delay that is
-        in effect will no longer be applied, and further non-essential
-        maintenance tasks (such as index vacuuming) are bypassed.
+        in effect will no longer be applied, further non-essential
+        maintenance tasks (such as index vacuuming) are bypassed, and any
+        <glossterm linkend="glossary-buffer-access-strategy">Buffer Access Strategy</glossterm>
+        in use will be disabled resulting in <command>VACUUM</command> being
+        free to make use of all of
+        <glossterm linkend="glossary-shared-memory">shared buffers</glossterm>.
        </para>
        <para>
         The default is 1.6 billion transactions.  Although users can