The aim of this WL is to implement CHECK constraints feature.
The feature is supported for all the storage engines.

The WL provides following functionality:

  a) Create check constraints on a table:

      WL implements following syntax to add check constraint in
      CREATE TABLE and ALTER TABLE,

        check_constraint_definition:
          [CONSTRAINT [symbol]] CHECK(expr) [[NOT] ENFORCED]

        Example:
          CREATE TABLE t1 (f1 INT CHECK (f1 < 929), f2 INT,
                           CHECK (f2 < 929));

          ALTER TABLE t1 ADD COLUMN f3 INT CHECK (f3 < 929),
                         ADD CONSTRAINT ck CHECK (f2 > 1);

      If enforcement is not specified then check constraint is
      created in the ENFORCED state.

   b) Check constraints validation:

       For DML operations INSERT, UPDATE, REPLACE, LOAD DATA,
       LOAD XML, check constraints in ENFORCED states are
       validated on each row. The check constraint is satisfied
       if and only if expression is evaluated to TRUE or
       UNKNOWN(for NULL column value). Constraint violation
       error is reported otherwise.

       Example:
        CREATE TABLE t (s1 INT, CHECK (s1 > 0))

	INSERT INTO t VALUES (-1)
	/* this should fail, the condition is FALSE */

	INSERT INTO t VALUES (NULL)
	/* this should succeed, the condition is UNKNOWN */

	INSERT INTO t VALUES (1)
	/* this should succeed, the condition is TRUE */

   c) Alter check constraint's state:

       WL implements following syntax in ALTER TABLE statement
       to alter constraint's state.

         ALTER TABLE ... ALTER CHECK symbol [[NOT] ENFORCED]

         Example:
           ALTER TABLE t1 ALTER CHECK ck NOT ENFORCED;

           ALTER TABLE t1 ALTER CHECK ck ENFORCED;

   d) Drop check constraint:

       WL implements following syntax in ALTER TABLE statement
       to DROP check constraint.

         ALTER TABLE ... DROP CHECK symbol;

         Example:
	   ALTER TABLE t1 DROP CHECK ck;

    e) SHOW CREATE TABLE lists check constraints defined on the
       table.

         Example:
          CREATE TABLE t1(
            f1 INT CHECK (f1 < 10),
            f2 INT CHECK (f2 < 10) NOT ENFORCED);

          SHOW CREATE TABLE t1;
            Table   Create Table
            t1      CREATE TABLE `t1` (
              `f1` int(11) DEFAULT NULL,
              `f2` int(11) DEFAULT NULL,
              CONSTRAINT `t1_chk_1` CHECK ((`f1` < 10)),
              CONSTRAINT `t1_chk_2` CHECK ((`f2` < 10)) /*!80015 NOT ENFORCED */,
            );

        All check constraints are listed in the table check constraint
        form.

        Check constraint in not enforced state are listed with the
        conditional comment "NOT ENFORCED".

    f) INFORMATION_SCHEMA.CHECK_CONSTRAINTS is introduced to list
       check constraints.

    g) INFORMATION_SCHEMA.TABLE_CONSTRAINTS now lists even check
       constraints defined on the table.

       Column "ENFORCED" is introduced to TABLE_CONSTRAINTS table
       to list enforcement state [YES/NO] for constraints.

    h) WL implements core features defined in the ANSI/ISO SQL
       standard E141-06 "CHECK CONSTRAINTS".

       Please refer WL's HLS section R for the common restrictions
       on the check constraints.

Change-Id: I9de7158200e829e05df322f8ffe7e782aa9b7ab1