Allow start of arbitrary XQuery during eXist-db startup.

Results are logged in exist.log.

usage:
```xml
 <startup>
    <triggers>
      <trigger class="org.exist.collections.triggers.XQueryStartupTrigger">
        <parameter name="xquery" value="/db/script1.xq"/>
        <parameter name="xquery" value="/db/script2.xq"/>
      </trigger>
    </triggers>
  </startup>
```