Update DUMP DDL command to use new CellSchema definitions.

AbstractDumpDDLCommand now dumps ALTER TABLE.. ADD SCHEMA statements.

In a layout-validating table, we now dump all schemas (Readers and
writers) associated with the table at creation time.

CREATE TABLE syntax is modified to allow omitting the schema definition.
You may now create "schemaless" columns and add readers and writers later.
ALTER TABLE { ADD | DROP } SCHEMA support schema ids.

Allow ALTER TABLE.. { ADD | DROP } SCHEMA commands to reference
schemas by id (... ADD SCHEMA ID <number> FOR COLUMN...).
Add DESCRIBE TABLE t COLUMN c SHOW SCHEMAS.

Adds a command to show schemas within a column of a table.
Add new syntax to help text in InputProcessor.

Also, warn the user if they remove the default reader schema.