Add Field.compare(Comparator, Select) and Field.compare(Comparator, QuantifiedSelect) to allow for more dynamic SQL

[lukaseder]

Similar to #1842, there should be a dynamic way to compare fields with subselects and quantified subselects. In order for this to be possible, the internal, package-private org.jooq.impl.SubqueryOperator type should be merged into the public org.jooq.Comparator type, adding new comparators (IN, NOT_IN) to the latter.

Some additional flags should be added to Comparator, in order to distinguish which comparator can be used:

• for "regular" comparison predicates
• for "subselect" comparison predicates
• for "quantified" comparison predicates