Modify the history_trades table to be friendlier for price aggregation

[tomerweller]

In preparation for #94, modifying the history_trades table can assist in simpler aggregation.
The main two issues to handle:

1. history_trades has no time of creation, only operation id. This requires joining with the history_ledgers to get exact times.
2. The separation of buyers and sellers is not relevant for aggregation and forces every query to handle both possible directions.

[tomerweller]

A proposition for this table, assuming there's a history_assets lookup_table. (stellar-deprecated/horizon#378)

CREATE TABLE history_trades (
  history_operation_id BIGINT NOT NULL,
  ledger_closed_at TIMESTAMP NOT NULL,
  "order" INTEGER NOT NULL,
  offer_id BIGINT NOT NULL,
  base_asset_id BIGINT NOT NULL REFERENCES history_assets(id),
  base_volume BIGINT NOT NULL CHECK (base_volume > 0),
  counter_asset_id BIGINT NOT NULL REFERENCES history_assets(id),
  counter_volume BIGINT NOT NULL CHECK (counter_volume > 0),
  direction BIT, -- 0: base is seller, 1: counter is seller
  CHECK(base_asset_id < counter_asset_id) -- Check that convention is followed.
);

CREATE INDEX htrd_pair_time_lookup ON history_trades USING BTREE(base_asset_id, counter_asset_id, ledger_closed_at);
CREATE INDEX htrd_time_lookup ON history_trades USING BTREE(ledger_closed_at);
CREATE INDEX htrd_counter_lookup ON history_trades USING BTREE(counter_asset_id);
-- Are the following indexes actually used?
CREATE UNIQUE INDEX htrd_pid ON history_trades USING btree (history_operation_id, "order");
CREATE INDEX htrd_by_offer ON history_trades USING btree (offer_id);