SI-9623 Avoid unnecessary hasNext calls in JoinIterator & ConcatIterator

These iterator implementations are used to concatenate two (JoinIterator)
or more (ConcatIterator) other iterators with `++`. They used to perform
many unnecessary calls to the child iterators’ `hasNext` methods.

This improved state machine-based implementation reduces that number to
the bare minimum, i.e. iterating over concatenated iterators with
`foreach` calls the children's `hasNext` methods a total of (number of
children) + (number of elements) times, the same as when iterating over
all children separately.

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