Metacat

In class ForceReplicationHandler, we use a for loop the check every server in
replication tables to see if we need to send a url force replication request.
But in the start of for loop, we didn't reset the url to null. So if the next
iteration didn't overwrite the previous one, the previouse one will send twice.
So the remote server will request a same document twise and cause the error.
Now the url will be reset again and the error between two replication of
metacat is gone.

But if there are 3 metacats - A, B, C and they can replicate to each other. If
A inert a document and it will replicate to B and C. B write the documents
into its database and send a force replication request to C. C will ask the
same doc form B. But when C got the doc info from and found it already have it.
So the exception will happend.

Same thing will happend to B too.

If we don't want the feature that force replication will happend during a
replication(if we do that, we will get rid of the exception). The hub will not
automatically replicate the documents which it got from other host to the
remote host.

It is only exception and no hurt to metacat. So we can keep it.

Original Bugzilla ID was 540