Improvements for Key Management Guide

[ConorOkus]

• Highlight the fact we expect users to implement their own KeysInterface but use InMemorySigner for channel-specific parts.
• Some recommendations for transferring state to a separate node/wallet implementation given, spending logic requires a lot of custom logic
• LDK will always require the user to provide the initial randomness from which things like this can be derived for the default KeysManager
• What gets persisted so that channel keys can be recovered on restart?
• What is needed to rederive keys
  • channel_keys_id for the Signer?
  • seed for KeysManager

[ConorOkus]

Update from Slack convo"

Jeff says: "That seed is used to create a master key, from which we derive other keys. Check out KeysManager::new to see how this is done. So if BDK already has an HD wallet with its own seed, you'd want to have a separate derivation path for LDK use (at least where Matt pointed out) rather than reusing the seed from BDK. But LDK uses the seed for other things, you'll notice."