Repeated crashes could result in the `last_processed_block` variable being
pushed further and further into the past (in some cases going as far back as
scanning blocks from 2012...). This is a stop-gap solution that just lower
bounds the value to what is the first possible block we might be interested in
LN, until we have the 0-rescan fix I'm working on.
Signed-off-by: Christian Decker <decker.christian@gmail.com>