[server] Add source_by_eid and source_by_uri methods to repository
Most of the times we only need to retrieve one source (either by uri or eid)
and querying sources_by_eid and sources_by_uri properties on repository just
for one item is costly. So these methods query what's needed. We issue a
ValueError (instead of KeyError for sources_by_{eid,uri} dict) in case the key
is not found.