script with _score: remove dependency of DocLookup and scorer

As pointed out in #7487 DocLookup is a variable that is accessible by all scripts
for one doc while the query is executed. But the _score and therfore the scorer
depends on the current context, that is, which part of query is currently executed.
Instead of setting the scorer for DocLookup
and have Script access the DocLookup for getting the score, the Scorer should just
be explicitely set for each script.
DocLookup should not have any reference to a scorer.
This was similarly discussed in #7043.

This dependency caused a stackoverflow when running script score in combination with an
aggregation on _score. Also the wrong scorer was called when nesting several script scores.

closes #7487
closes #7819
field name lookup: return List instead of Set for names matching a pattern

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>

closes #7709