In this commit, we convert the existing `channeldb.ChannelType` type
into a _bit field_. This doesn't require us to change the current
serialization or interpretation or the type as it is, since all the
current defined values us a distinct bit. This PR lays the ground work
for any future changes that may introduce new channel types (like anchor
outputs), and also any changes that may modify the existing invariants
around channels (if we're the initiator, we always have the funding
transaction).
In this commit, we create a new Single version for channels that use the
tweakless commitment scheme. When recovering from an SCB into an open
channel shell, we'll now check this field and use it to determine the
proper channel type. Otherwise, we may attempt to sweep the on chain
funds using the commitment point, when it goes directly to our key, or
the other way around.
In this commit, we extend the prior Single format to include the entire
channel config, other than the constraints, but including the CSV delay
for both sides. We do this as we'll need more of the keying information
in order to properly execute the DLP protocol. Additionally, in the
future, if warranted, this would allow channels to be resumed if deemed
safe.
In this commit, we add the initial implementation of the SCB structure.
Given an SCB, and a user's seed, it will be possible to recover the
settled balanced of a channel in the event of total or partial data
loss. The SCB contains all information required to initiate the data
loss protection protocol once we restore the channel and connect to the
remote channel peer.
The primary way outside callers will interact with this package are via
the Pack and Unpack methods. Packing means writing a
serialized+encrypted version of the SCB to an io.Writer. Unpacking does
the opposite.
The encoding format itself uses the same encoding as we do on the wire
within Lightning. Each encoded backup begins with a version so we can
easily add or modify the serialization format in the future, if new
channel types appear, or we need to add/remove fields.
