Added name to MEM_ROOT for esier debugging

This will make it easier to how memory allocation is done when debugging
with either DBUG or gdb.

Will especially help when debugging stored procedures

Main change is a name argument as second argument to init_alloc_root()
init_sql_alloc()

Other things:
- Added DBUG_ENTER/EXIT to some Virtual_tmp_table functions

sql/sql_class.h

@@ -1791,7 +1791,8 @@ public:
     m_reopen_array(NULL),
     m_locked_tables_count(0)
   {
-    init_sql_alloc(&m_locked_tables_root, MEM_ROOT_BLOCK_SIZE, 0,
+    init_sql_alloc(&m_locked_tables_root, "Locked_tables_list",
+                   MEM_ROOT_BLOCK_SIZE, 0,
                    MYF(MY_THREAD_SPECIFIC));
   }
   void unlock_locked_tables(THD *thd);
@@ -2540,7 +2541,8 @@ public:
     {
       bzero((char*)this, sizeof(*this));
       xid_state.xid.null();
-      init_sql_alloc(&mem_root, ALLOC_ROOT_MIN_BLOCK_SIZE, 0,
+      init_sql_alloc(&mem_root, "THD::transactions",
+                     ALLOC_ROOT_MIN_BLOCK_SIZE, 0,
                      MYF(MY_THREAD_SPECIFIC));
     }
   } transaction;