[feat][refactor] Methods for tbl cols and col default (#573)
[feat][refactor] Methods for tbl cols and col default (#573)

This adds a helper method to get the table's column definitions, caching
it to avoid multiple database hits, and a method for getting the default
value of a column. In adding these methods, some refactoring to simplify
code.
hubzero/framework,hubzero/framework