Simply sync flow by removing the previously added validation thread. All parts of sync now run on a single stateful sync thread as before, simplifying logic.
Adds more granular state messages and tracking to step 3/7 (preparing chain state for validation) where possible
Propagate stop_state to ensure ctrl+c can be used throughout most of the validation process
Add a step to the PIBD request process to clear out requested blocks that haven't been received within 15 seconds (will probably need to adjust this)
Changes to tx rangeproof validation function in anticipation of allowing the validation process to be stopped and resume where it left off.