Create simple asset table

[jedmccaleb]

As a beginning, and to help with this issue: #103. Maybe we should start with a simple compact history_assets table that will serve as a lookup table, something like this:

CREATE TABLE history_assets (
  id SERIAL PRIMARY KEY ,
  asset_type VARCHAR(64) NOT NULL,
  asset_code VARCHAR(12) NOT NULL,
  asset_issuer VARCHAR(56) NOT NULL,
  UNIQUE(asset_type, asset_code, asset_issuer)
);