v0.5.0: Fixed nomination timer, improved flash API

• Network discovery period is now configurable (node.network_discovery_interval_secs);
• Nomination interval is now configurable (validator.nomination_interval, in seconds);
• Nomination interval is now 5s by default, up from 1s; this should improve liveness;
• Flash can now use a registry;
• Flash API now includes onRequestedChannelOpen to query if a node will accept a channel open;
• Fixed a race condition preventing envelopes with valid signature to be accepted after externalization;
• Periodic catchup will now longer ask for signatures from slashed nodes;
• Flash: changeFee was only implemented in test, moved to production;
• Vanity miner now supports Bech32, and well-known keypairs have been changed accordingly;
• The node will now stop nominating as soon as it starts shutting down;