escape whole word operators (OR, AND, NOT) [LUCENE-6221]

The current QueryParser escape method does not escape whole word operators (OR, AND, NOT):

```Java
public static String escape(String s) {
StringBuilder sb = new StringBuilder();
for (int i = 0; i < s.length(); i++) {
char c = s.charAt(i);
// These characters are part of the query syntax and must be escaped
if (c == '\\' || c == '+' || c == '-' || c == '!' || c == '(' || c == ')' || c == ':'
|| c == '^' || c == '[' || c == ']' || c == '\"' || c == '{' || c == '}' || c == '\~'
|| c == '*' || c == '?' || c == '|' || c == '&' || c == '/') {
sb.append('\\');
}
sb.append(c);
}
return sb.toString();
}
```

It would be better if these words were escaped too.

---
Migrated from [LUCENE-6221](https://issues.apache.org/jira/browse/LUCENE-6221) by Shaun A Elliott

The reported entry point is QueryParser.escape(String); locate its Java source and inspect how whole-word operators are parsed. Confirm the expected escaped representation for OR, AND, and NOT, then verify the change with the relevant existing QueryParser tests.

Written by the indexing model from the issue text.