Provide vectored variant of ReadBuffer().

Break ReadBuffer() up into two steps.  StartReadBuffers() and
WaitReadBuffers() give us two main advantages:

1.  Multiple consecutive blocks can be read with one system call.
2.  Advice (hints of future reads) can optionally be issued to the
kernel ahead of time.

The traditional ReadBuffer() function is now implemented in terms of
those functions, to avoid duplication.

A new GUC io_combine_limit is defined, and the functions for limiting
per-backend pin counts are made into public APIs.  Those are provided
for use by callers of StartReadBuffers(), when deciding how many buffers
to read at once.  The following commit will add a higher level mechanism
for doing that automatically with a practical interface.

With some more infrastructure in later work, StartReadBuffers() could
be extended to start real asynchronous I/O instead of just issuing
advice and leaving WaitReadBuffers() to do the work synchronously.

Author: Thomas Munro <[email protected]>
Author: Andres Freund <[email protected]> (some optimization tweaks)
Reviewed-by: Melanie Plageman <[email protected]>
Reviewed-by: Heikki Linnakangas <[email protected]>
Reviewed-by: Nazir Bilal Yavuz <[email protected]>
Reviewed-by: Dilip Kumar <[email protected]>
Reviewed-by: Andres Freund <[email protected]>
Tested-by: Tomas Vondra <[email protected]>
Discussion: https://postgr.es/m/CA+hUKGJkOiOCa+mag4BF+zHo7qo=o9CFheB8=g6uT5TUm2gkvA@mail.gmail.com

Author: macdice

doc/src/sgml/config.sgml

@@ -2708,6 +2708,20 @@ include_dir 'conf.d'
        </listitem>
       </varlistentry>
 
+      <varlistentry id="guc-io-combine-limit" xreflabel="io_combine_limit">
+       <term><varname>io_combine_limit</varname> (<type>integer</type>)
+       <indexterm>
+        <primary><varname>io_combine_limit</varname> configuration parameter</primary>
+       </indexterm>
+       </term>
+       <listitem>
+        <para>
+         Controls the largest I/O size in operations that combine I/O.
+         The default is 128kB.
+        </para>
+       </listitem>
+      </varlistentry>
+
       <varlistentry id="guc-max-worker-processes" xreflabel="max_worker_processes">
        <term><varname>max_worker_processes</varname> (<type>integer</type>)
        <indexterm>