- Adds the SECONDARY_ENGINE, SECONDARY_LOAD, SECONDARY_UNLOAD keywords.

 - Adds a mock storage engine for testing the new syntax without
 depending on an actual secondary engine.

 - Adds the test suite "secondary_engine" to the default set of test
 suites.

For example, to define a secondary engine for a table:

    CREATE TABLE t1 (a INT) SECONDARY_ENGINE <engine_name>;
or
    ALTER TABLE t1 SECONDARY_ENGINE <engine_name>;

To load a table into its defined secondary engine:

    ALTER TABLE t1 SECONDARY_LOAD;

To unload a table from its secondary engine:

    ALTER TABLE t1 SECONDARY_UNLOAD;
or
    ALTER TABLE t1 SECONDARY_ENGINE NULL;

Change-Id: Id34dc5736f519f3fa9bca7449a48dd793bf40eeb