In this commit, we introduce a new options pattern for configuring
iterator behavior in the graph database. This includes configuration
for batch sizes when iterating over channel and node updates, as well
as an option to filter for public nodes only.
The new functional options pattern allows callers to customize iterator
behavior without breaking existing APIs. Default batch sizes are set to
1000 entries for both channel and node updates, which provides a good
balance between memory usage and performance.
In this commit, we let the ChannelGraph be responsible for populating
the graphCache and then passing it to the KVStore. This is a first step
in moving the graphCache completely out of the KVStore layer.
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.
