Allow topics to be shared between servers

At the moment topics can only service one server with multiple instances. If you try to add a topic to 2 different servers the XSD parsing fails with a message that a service must be unique to one server.

Agreed that generally this should be the case, but for topics it would be advantageous for all servers to be able to subscribe to a topic, as although the logic is mostly different some of ours share functionality, and being able to send the same info in one call to multiple servers is useful.

eg current functionality is allowed:

<server name='foo'>
<machine-ref id='fooapp1'/>
<machine-ref id='fooapp2' />
<service id='topica' type='topic' />
</server>

but I would like this to be possible:

<server name='foo'>
<machine-ref id='fooapp1'/>
<machine-ref id='fooapp2' />
<service id='topica' type='topic' />
</server>

<server name='bar'>
<machine-ref id='barapp1'/>
<machine-ref id='barapp2' />
<service id='topica' type='topic' />
<service id='barsvc'/>
</server>