Add debugging to navdata/save.

To help track down #221.

lib/www/server/lib/db/navdata/save.js

@@ -1,5 +1,7 @@
 // FIXME This code is in painful need of refactoring
 
+const debug = require("DOUGAL_ROOT/debug")(__filename);
+const debug_candidates = debug.extend("candidates");
 const { setSurvey, transaction, pool } = require('../connection');
 
 let last_tstamp = 0;
@@ -253,6 +255,7 @@ async function save (navData, opts = {}) {
 			});
 			return obj;
 		}).filter(c => !!c);
+		debug_candidates("%j", candidates);
 // 		console.log("CANDIDATES", candidates);
 
 		if (candidates.length == 0) {