* Update `ForkedChainRef` constructor
why:
Initialisation is based on the canonical head which is always zero
after resuming a stopped `ForkedChainRef` based import.
* Update new-base calculator
why:
There is some ambiguous code which might not do what the comment
implies. In short, an unsigned condition like `2u - 3u < 1u => false`
is coded where the comment suggests that `2 - 3 < 1 => true` is meant.
This patch fixes notorious crashes when resuming import after a stop.
7d41a992e6