MDEV-8010 - Avoid sql_alloc() in Items (Patch #1)

Added mandatory thd parameter to Item (and all derivative classes) constructor.
Added thd parameter to all routines that may create items.
Also removed "current_thd" from Item::Item. This reduced number of
pthread_getspecific() calls from 290 to 177 per OLTP RO transaction.

sql/protocol.h

@@ -139,6 +139,10 @@ public:
   virtual enum enum_protocol_type type()= 0;
 
   void end_statement();
+
+  friend int send_answer_1(Protocol *protocol, String *s1, String *s2,
+                           String *s3);
+  friend int send_header_2(Protocol *protocol, bool for_category);
 };