field name lookup: return List instead of Set for names matching a patte...

...rn


The returned sets are only used for iterating. Therefore we might
as well return a list since this guaratees order.

This is the same effect as in
https://github.com/elasticsearch/elasticsearch/pull/7698
The test SimpleIndexQueryParserTests#testQueryStringFieldsMatch
failed on openjdk 1.7.0_65 with
<jdk.map.althashing.threshold>0</jdk.map.althashing.threshold>

Unsure if this is the best solution - we might as well just change the test SimpleIndexQueryParserTests#testQueryStringFieldsMatch