fix: ICori::do_getMetrics must correctly handle getting a request for a metric it doesn't have and return an empty Attributes. Because when more than one ICori are registered, any metric is requested to all ICori. Only one has the metric, others should not crash.
add some test cases for merging empty Attributes
Attributes: fix ptree_merge::operator() crashing when merging an empty ptree
add PilgrimCori (also adds dependency on libcurl)