Support price feeds from SDEX

[nikhilsaraf]

Desired Behavior

I want to be able to use price feeds from within SDEX itself.

Impact

The desired behavior will allow me to have one asset that follows the price of another asset on SDEX.

Feature Suggestion

We can achieve the desired behavior by having an sdex type of price feed where it looks at the orderbook within SDEX to get the ticker price.

Specification

This should ideally use a new instance of SDEX for the price feed which is instantiated without any of the secret key data from the original price feed, i.e. it will be a read-only instance of sdex.go

[nikhilsaraf]

the GetOrderBook method being introduced in this issue will unblock #34