Stable release v0.0.8

This release refers to the network stability and validator set healthiness. This is a major upgrade which is mandatory for everyone.

Features and improvements:

1. ImOnline trait implementation for Validators:
2. ImOnline heartbeat messages (sending/receiving by validators via off-chain worker)
3. tracking validators online, offline detection
4. kicking off the validators detected as offline
5. auto slashing 20 000 P3D penalties for being offline
6. Treasury trait setting up for all penalties:
7. system account (treasury pot) for penalties
8. spending funds from the treasury pot by Council voting
9. GRANDPA RPC calls for tracking finalization state:
10. actual voting state
11. votes missing
12. Auto re-lock option for validators (locks funds automatically every N blocks in the future)
13. Tracking funds locked up in a collateral:
14. new lock method for validator set
15. db migration, which allows to track funds locked by validators
16. kicking off the validators not having funds enough locked up
17. min lock time: 1 month (43200 blocks)
18. Comeback opportunity window for validators headed off the validator set, during which the validator can come back without having a block mined:
19. ban period: 3 hours
20. comeback window: 1 week
21. new method rejoinValidator to rejoin the validator set after getting kicked off
22. Validation Session:
23. reduced to 30 blocks
24. Council:
25. new method for Council to add a validator not having a block mined, which executes by voting
26. PoScan:
27. block verification improvements
28. blockchain db migration
29. Released online at the block height of 183736
30. not compatible with all previous versions

Full Changelog: https://github.com/3Dpass/3DP/compare/v0.0.7...v0.0.8