Fix: infinite ping-pong of keyblock when non-valid block
Update Protocol.md: add keychain/parameters & contract/parameters methods
Update Protocol.md: changing `powPeriod` parameter interpretation
Updated `powPeriod` interpretation in code
Implemented method keychain/parameters
Added PoW delay parameter before each PoW computation (optional)
Sync: add a question to trust remote's key for sync session
HTTP API: keychain/parameters should return the currency name
Sync: update UCP parameters to match the synced node's parameters
Updating vucoin dependency to 0.20.0