Refactor ChainDB q-s-m test to carry gap blocks between commands

Extends the ChainDB model with generator state to support carrying gap
blocks in state machine tests. This increases the chances of generating
and adding (possibly out-of-order) branching sequences of blocks. This,
in turn increases the chances of observing the event where the
chain selection logic switches from a longer to a shorter (but heavier)
chain containing a boosted block.

Co-authored-by: Agustin Mista <[email protected]>
Co-authored-by: Alexander Esgen <[email protected]>
Co-authored-by: Georgy Lukyanov <[email protected]>
Co-authored-by: Thomas BAGREL <[email protected]>
Co-authored-by: Nicolas BACQUEY <[email protected]>

Author: 5 people