Motivation:
To use NFSServerV41 with spring a constructor argument injection is
required. This makes spring configuration more complicated, as some
arguments are optional and **null** values have to be injected.

Modification:
Add NfsServerFactory, which implements Spring's FactoryBean and enables
simpler integration, for instance where device manager must null:

```
    <bean id="nfs-v41" class="org.dcache.nfs.v4.NFSServerV41Factory">
        <description>NFSv4.1 MDS server</description>
        <property name="operationFactory" ref="mds-factory"/>
        <property name="vfs" ref="vfs"/>
        <property name="exportFile" ref="export"/>
     </bean>
```

instead of:

```
    <bean id="nfs-v41" class="org.dcache.nfs.v4.NFSServerV41">
        <description>NFSv4.1 MDS server</description>
        <constructor-arg ref="mds-factory"/>
        <constructor-arg index="1"><null/></constructor-arg>
        <constructor-arg ref="vfs" />
        <constructor-arg ref="export"/>
     </bean>
```

Result:
Simpler integration with Spring.

Acked-by: Paul Millar
Target: master