MCOL-5603 Online alter can break data consistency

Propagate the TL lock to TL_READ_NO_INSERT, blocking any concurrent writes until the end of a copy stage.

After that, this lock will be unlocked, allowing a window to write into a table before MDL is upgraded to exclusive (see online_alter_read_from_binlog call and the "alter_table_online_before_lock" debug syncpoint). This window will be covered by the second replication iteration after the lock upgrade

dbcon/mysql/ha_mcs.cpp

@@ -922,6 +922,10 @@ THR_LOCK_DATA** ha_mcs::store_lock(THD* thd, THR_LOCK_DATA** to, enum thr_lock_t
 #ifdef INFINIDB_DEBUG
   puts("store_lock");
 #endif
+  if (thd_sql_command(thd) == SQLCOM_ALTER_TABLE
+      && thd_tx_isolation(thd) == ISO_REPEATABLE_READ 
+      && lock_type == TL_READ)
+    table->reginfo.lock_type= TL_READ_NO_INSERT;
   return to;
 }