Include operations from external pool in received order

Context

Fix #2866 (closed): when using an external operations pool, there was no guarantee which operations would be included in the block. This fix makes sure that operations are considered for inclusion in the order they are received from the external pool.

I have added some tests for this feature but also remove 1 test in baking.ml.

The removed test was indirectly relying on the external pool being unordered and cannot be replaced by using a regular mempool. It is only commented out at this point since we might want to reintegrate it further down the road.

Checklist

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