Corrected the XTraceMetadata propagation in the allocate RPC call, by
adding metadata to each of the add/release requests.  Allocate is used
as a heartbeat mechanism and as such previously was acting as an
unwanted fan-in/fan-out point.
Was inadvertently passing null collections to Collection.addAll. 
Added an additional XTraceContext.getThreadContext method to XTrace to
support the semantics we're looking for here.