Plain SQL API should recognise multi-character PostgreSQL ltree operators

[lukaseder]

Similar to JSON operators (#5307), ltree operators also include ? characters, which must not be handled as bind variables in plain SQL strings.

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See:

• Avoid parsing ? in plain SQL if followed immediately by another operator-like character in PostgreSQL #5307
• https://groups.google.com/forum/#!topic/jooq-user/bz0QjXBcNcE