Assert that b->U->first == 0.
Assert that b->U->first == 0.

The first step to remove the BUNfirst() macro and all the complication
that introdudes because of different code use in gdk is to assert that
b->U->first is always 0. This assertion is not triggered anywhere during
the sql testing, but is been triggered in 3 tests in mserver5. Namely,
gdkTests/bat_insert gdkTests/reload and gdkTests/void will trigger
the assertion and that is most likely because of the bat.delete function
call. If we decide to go through and clean the code from BUNfirst,
b->U->first, offset, etc. then we have to probably remove the delete
functionality, but that sounds ok because no one is using it anyway.

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