Emit events on important state updates

[detectivekim]

Description

In changes of important state variables, especially in maintenance functions, it is important to emit an event so that the contract can be easily tracked via indexers. Consider adding events at least in the following functions:

• setFallbackOracle()
• setCollateralAllowance()
• setTreasury()
• setCouponMarket()

[detectivekim]

setFallbackOracle() added at #100