v0.9.0: Rat Poison Squared on Steroids

We're pleased to announce the 0.9.0 release of c-lightning, named by Sebastian Falbesoner. This is a major release and includes new features, new integrations as well as major internal refactorings facilitating extensions and future developments. # Highlights for Users - We can now send multi-part payments as well as keysend spontaneous payments. This complements the support to receive these payments introduced in an earlier release. - Large payments will now be split automatically into multiple parts that are more likely to succeed right away, and better protect your privacy. - Should a payment not succeed at first we will adaptively split the amount and retry. - We added support for watchtower plugins that help protect the node's funds while the node is not currently connected to the network. # Highlights for the Network - Support for optional fields in some messages has been removed, reducing complexity and replacing them with the more modern TLV extensions - Splitting payments into smaller parts with MPP increases the traffic on the network and improves privacy by making payments look more homogenous. The added traffic provides cover for other payments. # Highlights for Developers - Extensible payment flow framework in `libplugin` allowing developers to write custom extensions without having to reinvent the wheel. - Added RPC methods to create, sign and send PSBT transactions allowing c-lightning to be better integrated with wallets and higher-level protocols, such as coin-join and dual-funding. - Added RPC methods to reserve and unreserve outputs controlled by the node to complement the PSBT support, for multi-step protocols. - Added extensive accounting support in the form of the `coin_movement` hook that reports any change in the funds controlled by the node. - Refactored the internals to use PSBT everywhere, leveraging `libwally` - A number of new hooks and notifications were added to the plugin interface to allow deeper integration with c-lightning More details can be found at https://github.com/ElementsProject/lightning/blob/v0.9.0/CHANGELOG.md Thanks to everyone for their contributions and bug reports; please keep them coming. Since 0.8.2, we've had 523 commits from 21 different authors. A special thanks goes to the 8 first time contributors: - Aiden McClelland @dr-bonez - Candle @CandleHater - Joe Miyamoto @joe.miyamoto - Michal Rostecki @mrostecki - Reza @rbndg - Sebastian Falbesoner @theStack - Vincenzo Palazzo @vincenzopalazzo - @mb300sd Cheers, Christian, Rusty, ZmnSCPxj, and Lisa