Fix index validation for nested $and

[willholley]

Overview

mango_selector:has_required_fields checks that a list of
indexed fields is covered by a given selector. The implementation
recurses through the selector, tracking fields that encounters.

Unfortunately, this skipped peers of combination operators. For
example,

"selector": {
	"$and":[
		"$and":[
			"A": "foo"
		],
		"$and":[
			"B": "bar"
		]
	]
}

would skip the first nested "$and" operator and only return "B"
as a covered field.

This commit explicitly handles this situation (the only combination
operator we care about is $and), so for the above selector we
would correctly identify "A" and "B" as covered fields.

Testing recommendations

Run the test suite. Test that Query selectors with nested $and operators correctly use json indexes that should cover them.

Related Issues or Pull Requests

Checklist

• Code is written and works correctly;
• Changes are covered by tests;
• Documentation reflects the changes;

[willholley]

Thanks to @afinne for identifying the issue. The original bug report showed that the index / query combination at https://pastebin.com/WiQ5Edgu failed.

[tonysun83]

+1