Add timestamp to block header

- Enables quickly deriving timestamp from header for log events

db/migrations/1531758090_create_headers_table.up.sql

@@ -3,6 +3,7 @@ CREATE TABLE public.headers (
   hash                  VARCHAR(66),
   block_number          BIGINT,
   raw                   JSONB,
+  block_timestamp       NUMERIC,
   eth_node_id           INTEGER,
   eth_node_fingerprint  VARCHAR(128),
   CONSTRAINT eth_nodes_fk FOREIGN KEY (eth_node_id)

db/schema.sql

@@ -1098,6 +1098,7 @@ CREATE TABLE public.headers (
     hash character varying(66),
     block_number bigint,
     raw jsonb,
+    block_timestamp numeric,
     eth_node_id integer,
     eth_node_fingerprint character varying(128)
 );