delete all custom fields during bulk update

A checkbox that allows to delete all custom field
values.
perform bulk cf update operations in order

Order of operations does matter especially when
there are values to add and delete at the same time.
It's more important with perl 5.18 where each walk
through hash can be in different order.

Ignore deletes if we're adding value for field that
can have only one value.
zebra stripes for custom fields in bulk updates
Merge branch '4.2/delete-all-cf-values-in-bulk' into 4.2-trunk