fix: blockchain sync issues

[CjS77]

closes #385 closes #386

Several edge cases and issues were resolved:

• block sync progress not updating
• handle edge cases where sync state reverts back a step or two

Also:

• DRY up several instances of essentially copying over unused intermediate structs.
• Clean up main command loop.
• Syncing with front-end devs, refactor returns JSON object to eliminate calculations on their side.
• Add STARTUP state so front-end can know when downloading actually begins.
• Add done field that indicates that chain is fully synced.

Example of response (this will get camelCase renamed into a JSON object):

 SyncProgressInfo { sync_type: Block, header_progress: 100, block_progress: 68, total_blocks: 79871, estimated_time_sec: 319, done: false }
...
SyncProgressInfo { sync_type: Block, header_progress: 100, block_progress: 99, total_blocks: 79871, estimated_time_sec: 261, done: false }
...
SyncProgressInfo { sync_type: Done, header_progress: 100, block_progress: 100, total_blocks: 79871, estimated_time_sec: 0, done: true }