Add Filters (#133)

* Add LogFilter struct

* Add log_filters table

* Add view for events watching

* Add cmd line "add_filter" to mimic eventual endpoint

* Allow multiple filters in config

db/migrations/1516648743_add_log_filters.down.sql

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+DROP TABLE log_filters;

db/migrations/1516648743_add_log_filters.up.sql

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+CREATE TABLE log_filters (
+  id         SERIAL,
+  name       VARCHAR NOT NULL CHECK (name <> ''),
+  from_block BIGINT CHECK (from_block >= 0),
+  to_block   BIGINT CHECK (from_block >= 0),
+  address    VARCHAR(66),
+  topic0     VARCHAR(66),
+  topic1     VARCHAR(66),
+  topic2     VARCHAR(66),
+  topic3     VARCHAR(66),
+  CONSTRAINT name_uc UNIQUE (name)
+);

db/migrations/1516653373_add_watched_event_logs.down.sql

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+DROP VIEW watched_event_logs;

db/migrations/1516653373_add_watched_event_logs.up.sql

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+CREATE VIEW block_stats AS
+  SELECT
+    max(block_number) AS max_block,
+    min(block_number) AS min_block
+  FROM logs;
+
+CREATE VIEW watched_event_logs AS
+  SELECT
+    log_filters.name,
+    logs.id,
+    block_number,
+    logs.address,
+    tx_hash,
+    index,
+    logs.topic0,
+    logs.topic1,
+    logs.topic2,
+    logs.topic3,
+    data,
+    receipt_id
+  FROM log_filters
+    CROSS JOIN block_stats
+    JOIN logs ON logs.address = log_filters.address
+                 AND logs.block_number >= coalesce(log_filters.from_block, block_stats.min_block)
+                 AND logs.block_number <= coalesce(log_filters.to_block, block_stats.max_block)
+  WHERE (log_filters.topic0 = logs.topic0 OR log_filters.topic0 ISNULL)
+        AND (log_filters.topic1 = logs.topic1 OR log_filters.topic1 ISNULL)
+        AND (log_filters.topic2 = logs.topic2 OR log_filters.topic2 ISNULL)
+        AND (log_filters.topic3 = logs.topic3 OR log_filters.topic3 ISNULL);