pull: Add --per-object-fsync

This is the opposite of
https://github.com/ostreedev/ostree/issues/1184

Motivated by OpenShift seeing etcd performance issues during
OS updates: https://github.com/openshift/machine-config-operator/issues/1897

Basically, if we switch to invoking `fsync()` as we go, it makes
ostree performance worse (in my tests, 31s to write 2G versus 7s if we
delay sync) but it avoids *huge* outliers in `fsync()` time for etcd.

man/ostree.repo-config.xml

@@ -127,6 +127,18 @@ Boston, MA 02111-1307, USA.
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><varname>per-object-fsync</varname></term>
+        <listitem><para>By default, OSTree will batch fsync() after
+        writing everything; however, this can cause latency spikes
+        for other processes which are also invoking fsync().
+        Turn on this boolean to reduce potential latency spikes,
+        at the cost of slowing down OSTree updates.  You most
+        likely want this on by default for "background" OS updates.
+        </para>
+        </listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><varname>min-free-space-percent</varname></term>
         <listitem>