Improve the algorithm for extracting multifields

[przemekwitek]

When extracting multifield, only one of the fields (i.e. the first aggregatable) should be extracted.

This should help to fix issues such as:
https://github.com/elastic/ml-team/issues/235
https://github.com/elastic/ml-team/issues/239
which data team raised

[elasticmachine]

Pinging @elastic/ml-core (:ml)