improve StateSync

[altergui]

today i merged an "MVP" of StateSync #1235

for next iteration i'm jotting down here a rough roadmap or wanted features:

• enable indexer in all nodes (miners, maybe even seed) so all produce snapshots
• statesync: better flag handling #1270
  • statesync enabled by default
  • hardcoded checkpoints (height,hash) for LTS (and possibly for stg, dev)
  • api endpoints to fetch the (height, hash) dinamically (a must for stg, dev)
• provide INFO level logs during statesync (AFAICS i'll need a small hack to increase the log verbosity of comet before starting statesync, and then decrease it after sync is finished 💩 )

for the deterministic snapshots, i've created #1289