Use Chainlink VRF V2 with Direct Funding Method

[hantuzun]

It's the time to integrate Chainlink VRF.

The documentations are available on:

• https://docs.chain.link/vrf/v2/direct-funding/examples/get-a-random-number/
• https://docs.chain.link/vrf/v2/security/

Here's the example contract getting randomness from Chainlink docs: https://remix.ethereum.org/#url=https://docs.chain.link/samples/VRF/VRFv2DirectFundingConsumer.sol

I suggest asking for the random number once in the startDrawPeriod function and recording a seed for the following reasons:

• The number draws will be faster. As VRF flows will require 2+ blocks for every number + the cooldown.
• The code will be simpler.
• We will know how much LINK to use.

If we agree on this, we'd only need to modify startDrawPeriod() and add fulfillRandomWords(...). fulfillRandomWords(...) could store the random word as a variable. And then we could use that variable and nonces in drawNumber() only by changing the variables we put into the hash functions.

Design considerations:

• The contract must be resilient to Chainlink VRF failing to call the fulfillRandomWords function.
• The contract must not change its random number once it is set. Otherwise, advanced players can change the number to increase their chances of winning.

@daggerhashimoto you should review this issue.