Replaced the multi-field type in favour for the multi fields option that can be set on any core field.

When upgrading to ES 1.0 the existing mappings with a multi-field type automatically get replaced to a core field with the new `fields` option.

If a `multi_field` type-ed field doesn't have a main / default field, a default field will be chosen for the multi fields syntax. The new main field type
will be equal to the first `multi_field` fields' field or type string if no fields have been configured for the `multi_field` field and in both cases
the default index will not be indexed (`index=no` is set on the default field).

If a `multi_field` typed field has a default field, that field will replace the `multi_field` typed field.

Closes to #4521