Feature: Hold the "latest confirmed oracle timestamp" in public storage on the BeaconChainOracle contract

[youfoundron]

User story
As a developer querying the BeaconChainOracle contract I wish to get the latest confirmed oracle timestamp from contract storage because filtering through event logs for this data is unnecessarily complex.

Actions

• Update the BeaconChainOracle contract to hold in storage the latestConfirmedOracleTimestamp
  • the contract would update this variable every time fulfillRequest was successfully called.