In this commit, we migrate all the code in `channeldb` to only reference
the new `kvdb` package rather than `bbolt` directly.
In many instances, we need to add two version to fetch a bucket as both
read and write when needed. As an example, we add a new
`fetchChanBucketRw` function. This function is identical to
`fetchChanBucket`, but it will be used to fetch the main channel bucket
for all _write_ transactions. We need a new method as you can pass a
write transaction where a read is accepted, but not the other way around
due to the stronger typing of the new `kvdb` package.
This commit loosens the fwdpkg reference acking to be more tolerant
of prior deletions. Specifically, we won't fail if certain channels
are not found or fwdpkgs do not exist. This will make us more
tolerant to future changes where we:
- remove fwdpkgs on channel close
- defensively cleanup stray responses
