UnifiedHighlighter: simplify "maxPassages" input API [LUCENE-7844]

The "maxPassages" input to the UnifiedHighlighter can be provided as an array to some of the public methods on UnifiedHighlighter. When it's provided as an array, the index in the array is for the field in a parallel array. I think this is awkward and furthermore it's inconsistent with the way this highlighter customizes things on a by field basis. Instead, the parameter can be a simple int default (not an array), and then there can be a protected method like `getMaxPassageCount(String field` that returns an Integer which, when non-null, replaces the default value for this field.

Aside from API simplicity and consistency, this will also remove some annoying parallel array sorting going on.

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Migrated from [LUCENE-7844](https://issues.apache.org/jira/browse/LUCENE-7844) by David Smiley (@dsmiley), updated May 30 2017
Attachments: [LUCENE_7844__UH_maxPassages_simplification.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-7844/LUCENE_7844__UH_maxPassages_simplification.patch)

Start by inspecting UnifiedHighlighter's public methods that accept maxPassages and the existing by-field customization hooks. Review the attached LUCENE_7844__UH_maxPassages_simplification.patch for context. Done means the APIs use a simple integer default, field-specific overrides use a protected getMaxPassageCount(String field) hook, and the parallel-array sorting is removed.

Written by the indexing model from the issue text.