In this commit, a new NodeRTx interface is added which represents
consistent access to a persisted models.LightningNode. The
ForEachChannel method of the interface gives the caller access to the
node's channels under the same read transaction (if any) that was used
to fetch the node in the first place. The FetchNode method returns
another NodeRTx which again will have the same underlying read
transaction.
The main point of this interface is to provide this consistent access
without needing to expose the `kvdb.RTx` type as a method parameter.
This will then make it much easier in future to add new implementations
of this interface that are backed by other databases (or RPC
connections) where the `kvdb.RTx` type does not apply.
We will make use of the new interface in the `autopilot` package in
upcoming commits in order to remove the `autopilot`'s dependence on the
pointer to the `*graphdb.ChannelGraph` which it has today.
Find and replace all nolint instances refering to the `lll` linter and
replace with `ll` which is the name of our custom version of the `lll`
linter which can be used to ignore log lines during linting.
The next commit will do the configuration of the custom linter and
disable the default one.
We also now use the graph DB's own optional functions. An instance of
the graph is currently still passed to the channeldb's
`CreateWithBackend` function. This will be removed in a later commit
once the two have been completely disjoint.
