Revert recovery prefetching feature.

This set of commits has some bugs with known fixes, but at this late
stage in the release cycle it seems best to revert and resubmit next
time, along with some new automated test coverage for this whole area.

Commits reverted:

dc88460c: Doc: Review for "Optionally prefetch referenced data in recovery."
1d257577: Optionally prefetch referenced data in recovery.
f003d9f8: Add circular WAL decoding buffer.
323cbe7c: Remove read_page callback from XLogReader.

Remove the new GUC group WAL_RECOVERY recently added by a55a9847, as the
corresponding section of config.sgml is now reverted.

Discussion: https://postgr.es/m/CAOuzzgrn7iKnFRsB4MHp3UisEQAGgZMbk_ViTN4HV4-Ksq8zCg%40mail.gmail.com

doc/src/sgml/wal.sgml

@@ -803,21 +803,6 @@
    counted as <literal>wal_write</literal> and <literal>wal_sync</literal>
    in <structname>pg_stat_wal</structname>, respectively.
   </para>
-
-  <para>
-   The <xref linkend="guc-recovery-prefetch"/> parameter can
-   be used to improve I/O performance during recovery by instructing
-   <productname>PostgreSQL</productname> to initiate reads
-   of disk blocks that will soon be needed but are not currently in
-   <productname>PostgreSQL</productname>'s buffer pool.
-   The <xref linkend="guc-maintenance-io-concurrency"/> and
-   <xref linkend="guc-wal-decode-buffer-size"/> settings limit prefetching
-   concurrency and distance, respectively.  The
-   prefetching mechanism is most likely to be effective on systems
-   with <varname>full_page_writes</varname> set to
-   <varname>off</varname> (where that is safe), and where the working
-   set is larger than RAM.  By default, prefetching in recovery is disabled.
-  </para>
  </sect1>
 
  <sect1 id="wal-internals">