lending: Support borrow / lending between two non-quote reserves

[jstarry]

Problem

When borrowing funds, one of the deposit / borrow reserves must be the quote currency reserve. This limits lending to pairs which always include the quote currency. For example, if the quote currency is USDC, then providing SRM collateral to borrow SOL is not supported.

Each non-quote reserve specifies a single dex market between its currency and the lending market quote currency. In order to discover the price between two non-quote currencies, token values must be converted to the common quote currency which requires traversing the order book from the dex markets specified by the two reserve.

Proposed Changes

Extend borrow / liquidate instructions to allow passing an additional pair of dex market / order book account. Then, discover exchange price by first determining the value of tokens in the quote currency.