Return multiple witnesses for conflict queries.

[bredelings]

When a tree places a single Bacterium is placed in Eukaryotes (for example), it is useful to know that the tree conflicts with both Bacteria and Eukaryota. The previous service would pick one.
The otctera web services currently return a single node ID, but a name constructed from multiple IDs.

This need coordination with the web-app to avoid breaking the conflict service. @bredelings can modify the web services to return a list of (witness,name) pairs. I hope that @jimallman could modify the curator app to consume such a list.