replace my_timer_microseconds with microsecond_interval_timer

As svoj points out, my_timer_microseconds uses gettimeofday which is
affected by "discontinuous jumps in the system time", according to
man.

This patch changes to use microsecond_interval_timer which is "not
perfect, but is less affected by these jumps", and also does not
require an include of my_rdtsc.h header.

https://github.com/MariaDB/server/pull/332#discussion_r114708923

client/mysql.cc

@@ -34,7 +34,6 @@
 #include <m_ctype.h>
 #include <stdarg.h>
 #include <my_dir.h>
-#include <my_rdtsc.h>
 #ifndef __GNU_LIBRARY__
 #define __GNU_LIBRARY__		      // Skip warnings in getopt.h
 #endif
@@ -5035,7 +5034,7 @@ void tee_putc(int c, FILE *file)
 
 static ulonglong start_timer(void)
 {
-  return my_timer_microseconds();
+  return microsecond_interval_timer();
 }