Problem :
---------
Information_Schema.referential_constraints (UNIQUE_CONSTRAINT_NAME)
shows NULL for a foreign key constraint after restarting the server.
If any dml or query (select/insert/update/delete) is done on
referenced table, then the constraint name is correctly shown.

Solution :
----------
UNIQUE_CONSTRAINT_NAME column is the key name of the referenced table.
In innodb, FK reference is stored as a list of columns in referenced
table in INNODB_SYS_FOREIGN and INNODB_SYS_FOREIGN_COLS. The referenced
column must have at least one index/key with the referenced column as
prefix but the key name itself is not included in FK metadata. For this
reason, the UNIQUE_CONSTRAINT_NAME is only filled up when the
referenced table is actually loaded in innodb dictionary cache.

The information_schema view calls handler::get_foreign_key_list() on
foreign key table to read the FK metadata. The UNIQUE_CONSTRAINT_NAME
information shows NULL based on whether the referenced table is
already loaded or not.

One way to fix this issue is to load the referenced table while reading
the FK metadata information, if needed.
Reviewed-by: Sunny Bains <sunny.bains@oracle.com>

RB: 14654