Bug#42643: InnoDB does not support replication of TRUNCATE TABLE

The problem was that TRUNCATE TABLE didn't take a exclusive
lock on a table if it resorted to truncating via delete of
all rows in the table. Specifically for InnoDB tables, this
could break proper isolation as InnoDB ends up aborting some
granted locks when truncating a table.

The solution is to take a exclusive metadata lock before
TRUNCATE TABLE can proceed. This guarantees that no other
transaction is using the table.

Incompatible change: Truncate via delete no longer fails
if sql_safe_updates is activated (this was a undocumented
side effect).

mysql-test/t/partition_innodb_semi_consistent.test

@@ -101,6 +101,7 @@ connection con2;
 --error ER_LOCK_WAIT_TIMEOUT
 UPDATE t1 SET b = 21 WHERE a = 1;
 --disable_info
+ROLLBACK;
 
 --echo # Switch to connection con1
 connection con1;
@@ -150,6 +151,7 @@ SELECT * FROM t1;
 connection con2;
 --reap
 SELECT * FROM t1;
+COMMIT;
 
 --echo # Switch to connection con1
 connection con1;