When performing a MultiSearch, we create child SearchRequests
from the original SearchRequest. In doing so we copy many fields.
But, copying of the SortOrder was incorrect, as this contains
state, and distint SortOrder objects must be used. This change
introduces a Copy() method to the SearchSort interface, and
to the SortOrder types. MultiSearch now creates a new copy of
the SortOrder for each child request.
a78e632bd6