This commit adds the ChannelFlags field, of type
lnwire.FundingFlags, to the OpenChannel struct,
including serialization for database storage.
This is done to preserve the flags that were
sent during channel opening, currently used
to determine whether a channel should be made
public or not after opening.
In this commit, we add a new files to the channeldb package: codec.go.
This file is similar to the way that serialization is done within the
lnwire package. The goal of this file is to reduce all the duplication
within the common serialization methods across the package.