Draft patch that fixes and a sketches test cases for:

Bug#20837 Apparent change of isolation level during transaction,
Bug#46527 COMMIT AND CHAIN RELEASE does not make sense,
Bug#53343 completion_type=1, COMMIT/ROLLBACK AND CHAIN don't 
preserve the isolation level
Bug#53346 completion_type has strange effect in a stored 
procedure/prepared statement

Make thd->tx_isolation mean strictly "current transaction 
isolation level"
Make thd->variables.tx_isolation mean "current session isolation
level".
The current transaction isolation level is now established
at transaction start. If there was a SET TRANSACTION
ISOLATION LEVEL statement, the value is taken from it.
Otherwise, the session value is used.
A change in a session value, made while a transaction is active,
whereas still allowed, no longer has any effect on the
current transaction isolation level. This is an incompatible
change.
A change in a session isolation level, made while there is
no active transaction, overrides SET TRANSACTION statement,
if there was any.
Changed the impelmentation to not look at @@session.completion_type
in the parser, and thus fixed Bug#53346.
Changed the parser to not allow AND NO CHAIN RELEASE,
and thus fixed Bug#46527.
Changed the transaction API to take the current transaction
isolation level into account:
- BEGIN/COMMIT now do preserve the current transaction
isolation level if chaining is on.
- implicit commit, XA COMMIT or XA ROLLBACK or autocommit don't.

sql/sql_class.h

@@ -2043,8 +2043,31 @@ public:
   uint	     server_status,open_options;
   enum enum_thread_type system_thread;
   uint       select_number;             //number of select (used for EXPLAIN)
-  /* variables.transaction_isolation is reset to this after each commit */
-  enum_tx_isolation session_tx_isolation;
+  /*
+    Current or next transaction isolation level.
+    When a connection is established, the value is taken from
+    @@session.tx_isolation (default transaction isolation for
+    the session), which is in turn taken from @@global.tx_isolation
+    (the global value).
+    If there is no transaction started, this variable
+    holds the value of the next transaction's isolation level.
+    When a transaction starts, the value stored in this variable
+    becomes "actual".
+    At transaction commit or rollback, we assign this variable
+    again from @@session.tx_isolation.
+    The only statement that can otherwise change the value
+    of this variable is SET TRANSACTION ISOLATION LEVEL.
+    Its purpose is to effect the isolation level of the next
+    transaction in this session. When this statement is executed,
+    the value in this variable is changed. However, since
+    this statement is only allowed when there is no active
+    transaction, this assignment (naturally) only affects the
+    upcoming transaction.
+    At the end of the current active transaction the value is
+    be reset again from @@session.tx_isolation, as described
+    above.
+  */
+  enum_tx_isolation tx_isolation;
   enum_check_fields count_cuted_fields;
 
   DYNAMIC_ARRAY user_var_events;        /* For user variables replication */