MDL-45725 core_tablelib Save user preferences for flexible tables

Signed-off-by: Tony Butler <tonyjbutler@gmail.com>
core_tablelib: Implement code changes suggested by Sam Hemelryk

Signed-off-by: Tony Butler <tonyjbutler@gmail.com>
MDL-45725 core_tablelib: Simplify user preferences code

Signed-off-by: Tony Butler <tonyjbutler@gmail.com>
MDL-45725 core_tablelib: Use json_en/decode and don't store in session

Signed-off-by: Tony Butler <tonyjbutler@gmail.com>