Move SortMode to org.elasticsearch.search and rename to MultiValueMode
example how to use decay for multi values

Relates to #3960
Multi value handling in decay functions

Decay functions currently only use the first value in a field that contains
multiple values to compute the distance to the origin. Instead, it should
consider all distances if more values are in the field and then use
one of min/max/sum/avg which is defined by the user.
remove dfeault from builder
another default remove
make multiFieldMode a MultiFieldMode instead of String
use ParseField instead of String
use StringBuilder
test
more test