In this commit, we define the SQL schemas for storing graph channel
data. This includes a new `channels` table and a new `channel_features`
table along with various indices.
In this commit, we add the `source_nodes` table. It points to entries in
the `nodes` table. This table will store one entry per protocol version
that we are announcing a node_announcement on.
With this commit, we can run the TestSourceNode unit test against our
SQL backends.
In this commit, the various SQL schemas required to store graph node
related data is defined. Specifically, the following tables are defined:
- nodes
- node_extra_types
- node_features
- node_addresses
