WL#3292: Foreign Keys & Cluster: NDB Cluster requirements for SQL-level foreign keys

NDB Cluster storage engine will implement the storage engine API for foreign key
support (WL#3288).

Additionally, the following is required:

In order to check consistency of the foreign key constraint defined via ALTER
TABLE, we need to make sure that name locks that are acquired for the time of
foreign key creation.
Possible solution: inform the cluster that there is only one mysqld server
that can modify the table at this given moment (acquire an exclusive lock on
the table).
Proper support for ALTER TABLE in Cluster is a pre-requirement for SQL
layer foreign keys to work properly with this storage engine.