IF IT CREATES PROBLEM STATES

If a DDL is executed from a MySQL Server, during an ongoing upgrade of
MySQL Cluster from a version that does not support MySQL Data Dictionary
to a version that does, the schema might become inconsistent across the
different nodes connected to the cluster, due to the following issues :

 1) If the datanodes are not upgraded yet, then the DDL from the upgraded
 MySQL Server would create the schema with the new extraMetadata
 information which will be ignored by the older data nodes. Due to this,
 the schema distribution doesn't work and the schema change won't be
 distributed to any upgraded or unupgraded MySQL Servers. And if the
 MySQL Server, where the DDL originated, is restarted after the DDL,
 the schema object might be lost.

 2) Any unupgraded MySQL Server connected to the cluster will miss out
 any schema changes made by a DDL from an upgraded MySQL Server as the
 older MySQL Server cannot read the newer extra metadata information.

This patch fixes these issues by blocking all DDLs in ndbcluster during
an upgrade, until all the nodes have been upgraded to a version that has
support for MySQL Data Dictionary. Any database related DDLs are still
allowed as they are executed in the Server layer and do not have these
issues.

DDLs are still allowed from the unupgraded MySQL Servers. They will
succeed in executing the DDL but the subsequent attempt to distribute
them to any upgraded MySQL Server will fail. The DDLs will eventually
get distributed to the upgraded MySQL Servers when the schema object
is being accessed or by the metadata sync thread if it is running.
They can also be synced by restarting the upgraded MySQL Server.

Change-Id: I7e61a6f6721a231dbf4d3cc505a0e1086fc466b7