fix: kyber pricefeed missing functions

[SeanJCasey]

The primary purpose of this PR is to re-implement a function that is defined in IPriceSource.sol and is used in Participation.sol, but is missing from KyberPriceFeed.sol, which caused the price feed to be unusable.

Additionally, it:

• makes KyberPriceFeed.sol inherit from IPriceSource.sol, and re-implements all other missing functions (even though these other functions are not used in the protocol codebase)
• reverts name of an event to its old name, to be consistent with the production version of the Melon subgraph
• replaces _failIfInvalid flag on update() with check that allows a 0 price to pass if the sanity rate (i.e., its expected off-chain rate) is exactly 0 (e.g., SAI does not have a Kyber reserve price, but it is in the list of registered assets)
• replaces the use of arbitrary reserveMin values in passing a srcQty to KyberNetworkProxy.getExpectedRate() with dynamic calculations of the src asset relative to a constant amount of weth (e.g., 0.01 WETH worth of DAI, 0.01 WETH worth of WBTC, etc).

[fubhy]

Closing this as we agreed to not pursue this further before the upcoming release. Will leave the branch in place in case we decide otherwise.