Simpler arena swapping code

Now thd->mem_root is a pointer to thd->main_mem_root and THR_MALLOC is a pointer to thd->mem_root.
This gives us the following benefits:
- Allow us to easily detect if arena has already been swapped before (this fixes a bug in setup_conds() where arena was swaped twice in some cases)
- Faster swaps of arenas (as we don't have to copy the whole MEM_ROOT)
- We don't anymore have to call my_pthread_setspecific_ptr(THR_MALLOC,...) to change where memory is alloced. Now it's enough to set thd->mem_root

sql/thr_malloc.cc

@@ -38,7 +38,7 @@ void init_sql_alloc(MEM_ROOT *mem_root, uint block_size, uint pre_alloc)
 
 gptr sql_alloc(uint Size)
 {
-  MEM_ROOT *root=my_pthread_getspecific_ptr(MEM_ROOT*,THR_MALLOC);
+  MEM_ROOT *root= *my_pthread_getspecific_ptr(MEM_ROOT**,THR_MALLOC);
   char *ptr= (char*) alloc_root(root,Size);
   return ptr;
 }