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.
In this commit, we add a new storage namespace to channeldb: the
ForwardingLog. This log will be used by higher level sub-systems to log
each successfully completed HTLC. Each payment circuit will be
summarized as a “ForwardingEvent”. A series of events can then be
queried via a time slice query. In a time slice query, the caller
specifies a time range, a number of events to skip, and the max number
of events to return. Each query will return the index of the final
item. As we have a max number of events we’ll return in a response,
callers may need to use this last offset index to seek further by
skipping that number of entries. Combining these fields, callers are
able to query the time series, skipping an arbitrary amount of events,
and capping the max number of returned events.
