[7.x] add span reparenting field child_ids (#3679)

[graphaelli]

Backports the following commits to 7.x:

• add span reparenting field child_ids (add span reparenting field child_ids #3679)

[axw]

Verified. First I sent a transaction ("tx") with three spans: parent, child, grandchild. Child and grandchild both set "parent_id" to parent, like in the inferred spans scenario:

{"metadata":{"service":{"name":"foo","agent":{"name":"bar","version":"1.0"}}}}                                                                                                                       
{"transaction":{"id":"e05278a871c22b01","name":"tx","type":"request","duration":1,"timestamp":1496170407154000,"span_count":{"dropped":0,"started":0},"trace_id":"e825a4a629a6d3738f7dc6283e3bc98a"}}
{"span":{"id":"8aedd6f0dc6769a0","timestamp":1496170407154000,"name":"parent","type":"app","duration":1,"parent_id":"e05278a871c22b01","trace_id":"e825a4a629a6d3738f7dc6283e3bc98a"}}               
{"span":{"id":"6c9764db5c348a43","timestamp":1496170407154000,"name":"grandchild","type":"app","duration":0.5,"parent_id":"8aedd6f0dc6769a0","trace_id":"e825a4a629a6d3738f7dc6283e3bc98a"}}         
{"span":{"id":"a1fe370ca4900421","timestamp":1496170407154000,"name":"child","type":"app","duration":0.5,"parent_id":"8aedd6f0dc6769a0","trace_id":"e825a4a629a6d3738f7dc6283e3bc98a"}}              

I then changed the "child" span event, adding the "grandchild" span's ID to it child IDs:

{"span":{"id":"a1fe370ca4900421","timestamp":1496170407154000,"name":"child","type":"app","duration":0.5,"parent_id":"8aedd6f0dc6769a0","trace_id":"e825a4a629a6d3738f7dc6283e3bc98a","child_ids":["6c9764db5c348a43"]}}

We can see in Discover that this value is indexed in the new child.id field:

And the waterfall looks as expected: