Fixed memory leaks in embedded server and mysqltest

This commit fixes the following issues:
- memory leak checking enabled for mysqltest. This cover all cases except
  calls to 'die()' that only happens in case of internal failures in
  mysqltest. die() is not called anymore in the result files differs.
- One can now run mtr --embedded without failures (this crashed or hang
  before)
- cleanup_and_exit() has a new parameter that indicates that it is called
  from die(), in which case we should not do memory leak checks. We now
  always call cleanup_and_exit() instead of exit() to be able to free up
  memory and discover memory leaks.
- Lots of new assert to catch error conditions
- More DBUG statements.
- Fixed that all results are freed in mysqltest (Fixed a memory leak in
  mysqltest when using prepared statements).
- Fixed race condition in do_stmt_close() that caused embedded server
  to not free memory. (Memory leak in mysqltest with embedded server).
- Fixed two memory leaks in embedded server when using prepared statements.
  These memory leaks caused timeout hangs in mtr when server was compiled
  with safemalloc. This issue was not noticed (except as timeouts) as
  memory report checking was done but output of it was disabled.

libmysqld/lib_sql.cc

@@ -266,6 +266,7 @@ static my_bool emb_read_prepare_result(MYSQL *mysql, MYSQL_STMT *stmt)
       mysql->server_status|= SERVER_STATUS_IN_TRANS;
 
     stmt->fields= mysql->fields;
+    free_root(&stmt->mem_root, MYF(0));
     stmt->mem_root= res->alloc;
     mysql->fields= NULL;
     my_free(res);
@@ -375,6 +376,7 @@ int emb_read_binary_rows(MYSQL_STMT *stmt)
     set_stmt_errmsg(stmt, &stmt->mysql->net);
     return 1;
   }
+  free_root(&stmt->result.alloc, MYF(0));
   stmt->result= *data;
   my_free(data);
   set_stmt_errmsg(stmt, &stmt->mysql->net);