db: set operations on paginated Data API iterators

[canepat]

Add support for intersection/union set operations on E3 Data API paginated iterators and change the paginated iterator API exposing has_next/next stream-like methods, notably:

PaginatedUint64& paginated = ...;
auto it = co_await paginated.begin();
while (const auto value = co_await it.next()) {
    // use *value;
}

Furthermore, conceptualise and improve PaginatedIterator abstraction and increase test coverage.

Extra

• cleanup RemoteTransactionTest

[battlmonstr]

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