Add "ProductID" To Symbol Struct

[zahin-mohammad]

We currently construct an exchange product (traded pair) from the raw base and raw quote. We should store the product pair as well as some exchanges have multiple product pairs for the same base/quoute (Kraken).

type Symbol struct {
        ProductID              string
    RawBase                string
    NormalizedBase         string
    RawQuote               string
    NormalizedQuote        string
}