text: Title: [APM] Change get transaction by trace id url
  Body: The current API to get the transaction based on the trace ID is `/api/apm/transaction/TRACE_ID`, but based on restful principals this is incorrect. 
  To make it consistent we should change it to `/api/apm/traces/{traceId}/root_transaction`, and move it to https://github.com/elastic/kibana/blob/master/x-pack/legacy/plugins/apm/server/routes/traces.ts#L19