Bike swap protocol 🚲 🔄

What's Changed

• add multikeysend endpoint by @kiwiidb in #241
• Add test for invoice list with zero amount invoices by @bumi in #245
• update 0 amount invoices with actual amount paid by @kiwiidb in #244
• fix http status header by @kiwiidb in #246
• GRPC server by @kiwiidb in #247
• respond with 500 if no payment sent by @kiwiidb in #251

Full Changelog: 0.11.0...0.12.0