ddfs xcat hangs if the first replicas are unavailable.

[pooya]

This is similar to issue #223. We can check the list of available nodes and only try to fetch a replica if it is known to disco.
This has also caused the GC to faile with the following message.

GC: stopping, unable to get tag <<"test">>: {error,timeout}

Therefore, it is not just a UX issue.

ddfs blobs test

yields the following:

disco://dev02/ddfs/vol0/blob/e5/test-0$57f-ea20f-9fd7f disco://dev01/ddfs/vol0/blob/e5/test-0$57f-ea20f-9fd7f disco://devdisco01/ddfs/vol0/blob/e5/test-0$57f-ea20f-9fd7f

And in this case, we had 6 nodes but we reduced it to only three nodes which means only devdisco01 is in the cluster now.

[pooya]

ddfs xcat test
took almost 6 minutes which is not acceptable.

[pooya]

In this scenario the job is also significantly slowed down.

[pooya]

The initial issue should be resolved with pull request #600. We still have to reproduce and fix the gc issue.