Fixed fetch subphase to not recreate the HitContext for each hit, so that the object cache stays the same

Fixed also bug in the fast vector highlighter which was raised by enabling the object cache, due to null FieldQuery (NPE) in case the objects are taken from the cache

Added tests to check if there are issues when highlighting multiple fields at the same time

Closes #4106
Trimmed down HighlighterSearchTests

Used common assertHighlight and concise mapping whenever possible
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