This commit adds a reset() closure to the kvdb.View function which will
be called before each retry (including the first) of the view
transaction. The reset() closure can be used to reset external state
(eg slices or maps) where the view closure puts intermediate results.
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 is a direct copy of the complete channeldb package. It only
changes the package declaration at the top of every file. We make this
full copy so that review can be focused on the actual changes made.
Otherwise changes may drown in all the file moves.
Linting for the new package is disabled, as it contains lots of
pre-existing issues.
