Better filter name for transient duration

[markoheijnen]

Currently all transient calls from the APIs have long names like 'wp_contributions_core_transient'. This should be change that all calls will use the filter 'wp_contributions_transient_duration' and that the filter will receive a second argument what the method is like:

apply_filters( 'wp_contributions_transient_duration', HOUR_IN_SECONDS * 12, 'changeset_items', 'wordpress' );