Update BlockChain to record NodeInfo (#95)

db/migrations/1512504078_add_nodes_table.down.sql

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+DROP TABLE nodes;

db/migrations/1512504078_add_nodes_table.up.sql

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+CREATE TABLE nodes (
+  id            SERIAL PRIMARY KEY,
+  genesis_block VARCHAR(66),
+  network_id NUMERIC,
+  CONSTRAINT node_uc UNIQUE (genesis_block, network_id)
+);

db/migrations/1512507280_add_node_fk_to_blocks.down.sql

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+ALTER TABLE blocks
+  DROP COLUMN node_id;

db/migrations/1512507280_add_node_fk_to_blocks.up.sql

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+ALTER TABLE blocks
+  ADD COLUMN node_id INTEGER NOT NULL,
+  ADD CONSTRAINT node_fk
+FOREIGN KEY (node_id)
+REFERENCES nodes (id);