Harvest: refactor deposit/borrow amount from sdk.Coin to sdk.Coins

[denalimarsh]

Idea: should we change Amount to sdk.Coins so that we can create only one borrow object even if multiple coins are borrowed. Would have to apply the same change to deposit. Seems like it might be worth it, as we're storing a lot of additional objects if people supply/borrow multiple coins.

Originally posted by @karzak in https://github.com/Kava-Labs/kava/pull/702#discussion_r513457806