Fix asynchronous binary metadata mappings

Any asynchronous binary metadata mapping never gets executed.

How to reproduce:

• start a 10.10 instance with nuxeo-dam install and the following XML extension defining an asynchronous binary metadata rule (and disabling the existing sync one):

  <require>org.nuxeo.ecm.platform.picture.binary.metadata.contrib</require>
  
  <extension point="metadataRules" target="org.nuxeo.binary.metadata" >
      <rule id="iptc" order="0" enabled="false" />
  </extension>
  
  <extension point="metadataRules" target="org.nuxeo.binary.metadata"v>
      <rule id="picture" order="0" enabled="true" async="true">
      <metadataMappings>
        <metadataMapping-id>IPTC</metadataMapping-id>
        <metadataMapping-id>EXIF</metadataMapping-id>
        <metadataMapping-id>XMP</metadataMapping-id>
      </metadataMappings>
      <filters>
        <filter-id>hasPictureFacet</filter-id>
      </filters>
    </rule>
  
  <extension point="filters" target="org.nuxeo.ecm.platform.actions.ActionService" >
    <filter id="hasPictureFacet">
      <rule grant="true">
        <facet>Picture</facet>
      </rule>
    </filter>
  </extension>
  

• create a Picture document with an image file
• change the value of its dc:description property to test (this should trigger the mapping defined on this property)
• download the image file attached to the Picture document

Expected result: metadata IPTC:Caption-Abstract in downloaded image file is set to test
Actual result: metadata IPTC:Caption-Abstract in downloaded image file is not set to test

You can check the image file's metadata with command exiftool -G -json <filename>.

If a breakpoint is set at https://github.com/nuxeo/nuxeo/blob/654f687f79e5a85bb29a83913f3e3d8e539176b3/addons/nuxeo-binary-metadata/src/main/java/org/nuxeo/binary/metadata/internals/listeners/BinaryMetadataWorkListener.java#L59, it is never reached.
Setting a breakpoint at https://github.com/nuxeo/nuxeo/blob/654f687f79e5a85bb29a83913f3e3d8e539176b3/addons/nuxeo-binary-metadata/src/main/java/org/nuxeo/binary/metadata/internals/listeners/BinaryMetadataWorkListener.java#L57 shows that variable execute is null.