Add KeysManager::derive_unique_start

Add KeysManager::derive_unique_start

In next commit, we add derive_channel_keys, a helper to rederive
keys from a SpendableOuptutDescriptor. We still need to ensure
that user rederive same set of keys even if KeyManager is started
at a later time. For doing so we need to pass starting time as
part of the re-derived sha_state, therefore we need to make it
accessible and part of channel state key material.

Seed is already required to be conserved between KeysManager
serializations.