Keep track of transaction commit timestamps

Transactions can now set their commit timestamp directly as they commit,
or an external transaction commit timestamp can be fed from an outside
system using the new function TransactionTreeSetCommitTsData().  This
data is crash-safe, and truncated at Xid freeze point, same as pg_clog.

This module is disabled by default because it causes a performance hit,
but can be enabled in postgresql.conf requiring only a server restart.

A new test in src/test/modules is included.

Catalog version bumped due to the new subdirectory within PGDATA and a
couple of new SQL functions.

Authors: Álvaro Herrera and Petr Jelínek

Reviewed to varying degrees by Michael Paquier, Andres Freund, Robert
Haas, Amit Kapila, Fujii Masao, Jaime Casanova, Simon Riggs, Steven
Singer, Peter Eisentraut

doc/src/sgml/ref/pg_resetxlog.sgml

@@ -22,6 +22,7 @@ PostgreSQL documentation
  <refsynopsisdiv>
   <cmdsynopsis>
    <command>pg_resetxlog</command>
+   <arg choice="opt"><option>-c</option> <replaceable class="parameter">xid</replaceable>,<replaceable class="parameter">xid</replaceable></arg>
    <arg choice="opt"><option>-f</option></arg>
    <arg choice="opt"><option>-n</option></arg>
    <arg choice="opt"><option>-o</option> <replaceable class="parameter">oid</replaceable></arg>
@@ -79,9 +80,12 @@ PostgreSQL documentation
   <para>
    The <option>-o</>, <option>-x</>, <option>-e</>,
    <option>-m</>, <option>-O</>,
+   <option>-c</>
    and <option>-l</>
    options allow the next OID, next transaction ID, next transaction ID's
-   epoch, next and oldest multitransaction ID, next multitransaction offset, and WAL
+   epoch, next and oldest multitransaction ID, next multitransaction offset,
+   oldest and newest transaction IDs for which the commit time can be retrieved,
+   and WAL
    starting address values to be set manually.  These are only needed when
    <command>pg_resetxlog</command> is unable to determine appropriate values
    by reading <filename>pg_control</>.  Safe values can be determined as
@@ -128,6 +132,19 @@ PostgreSQL documentation
      </para>
     </listitem>
 
+    <listitem>
+     <para>
+      A safe value for the oldest transaction ID for which the commit time can
+      be retrieved (first part of <option>-c</>) can be determined by looking
+      for the numerically smallest file name in the directory
+      <filename>pg_committs</> under the data directory.  Conversely, a safe
+      value for the newest transaction ID for which the commit time can be
+      retrieved (second part of <option>-c</>) can be determined by looking for
+      the numerically greatest file name in the same directory.  As above, the
+      file names are in hexadecimal.
+     </para>
+    </listitem>
+
     <listitem>
      <para>
       The WAL starting address (<option>-l</>) should be