Add light sync command

- Only syncs block headers (excludes block bodies, transactions, receipts, and logs)
- Modifies validation window to include the most recent block
- Isolates validation window to the variable defined in the cmd directory (blocks
  have a separate variable defined in the block_repository for determining when
  to set a block as final)

db/migrations/1531758090_create_headers_table.down.sql

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+DROP TABLE public.headers;

db/migrations/1531758090_create_headers_table.up.sql

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+CREATE TABLE public.headers (
+  id                    SERIAL PRIMARY KEY,
+  hash                  VARCHAR(66),
+  block_number          BIGINT,
+  raw                   bytea,
+  eth_node_id           INTEGER,
+  eth_node_fingerprint  VARCHAR(128),
+  CONSTRAINT eth_nodes_fk FOREIGN KEY (eth_node_id)
+  REFERENCES eth_nodes (id)
+  ON DELETE CASCADE
+);