This is a step toward removing the Chain::Lock class and reducing cs_main
locking.
This change affects behavior in a few small ways.
- If there's no max_height specified, percentage progress is measured ending at
wallet last processed block instead of node tip
- More consistent error reporting: Early check to see if start_block is on the
active chain is removed, so start_block is always read and the triggers an
error if it's unavailable