Fix Job module options

- Changed the getter method name for the property
"restartable" in JdbcHdfsOptionsMetadata to "getRestartable"
so that Metadata BeanWrapper's PropertyDescriptor sets
the read method name correctly.

- Initialize `tableName`, `columns` and `sql` properties with empty text
so that their placeholders in jdbchdfs.xml get set. This doesn't do any
harm as NamedColumnJdbcItemReader checks it again.

- Changed the return type of the boolean getter methods to primitive type
boolean in some of the job module options metadata pojos.