Simplification of the offset calculation to use the rescan parameter, and rename
of `wallet_first_blocknum`. We now use either relative rescan from our last
known location, or absolute if a negative rescan was given. It's all handled in
a single location (except the case in which the blockcount is below our
precomputed offset), so this should reduce surprises.
Signed-off-by: Christian Decker <decker.christian@gmail.com>
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