stats: create an enable phase, and a prepare phase.

Enable more or less takes the place of check, but also allows stats to
do some other configuration.  Prepare moves all of the code that readies
a stat for dumping into a separate function in preparation for supporting
serialization of certain pieces of statistics data.
While we're at it, clean up the visitor code and some of the python code.

src/python/swig/stats.i

@@ -48,7 +48,8 @@ void initMySQL(std::string host, std::string database, std::string user,
 
 void StatEvent(bool dump, bool reset, Tick when = curTick, Tick repeat = 0);
 
-void check();
+void enable();
+void prepare();
 void dump();
 void reset();