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:

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

Full Changelog: v0.0.7...v0.0.8