Add filter cache metadata for relationships.

Attached patch, no tests. Also moved $base_table_view = $config->get('base_table'); since is only need outside of foreach loop.

Note: If filter is in an exposed block, then #3067937: Views Exposed Filter Block not inheriting the display handlers cache tags, causing filter options not to appear → needs to be applied.

The issue will be checked in 2.0.x-dev branch.

Thanks for the patch @silverham

The tests will be added with beta3

Automatically closed - issue fixed for 2 weeks with no activity.