C4 audit mitigations

[PierrickGT]

This PR fixes the following issues:

• Gas optimization on redeemToken of ATokenYieldSource code-423n4/2021-06-pooltogether-findings#123
• Gas optimization on _depositToAave code-423n4/2021-06-pooltogether-findings#122
• Lack of nonReentrant modifier in yield source contracts code-423n4/2021-06-pooltogether-findings#119
• Gas savings on uninitialized variables. code-423n4/2021-06-pooltogether-findings#101
• ATokenYieldSource save gas with pre-approval code-423n4/2021-06-pooltogether-findings#93
• ATokenYieldSource mixes aTokens and underlying when redeeming code-423n4/2021-06-pooltogether-findings#86
• Missing parameter validation code-423n4/2021-06-pooltogether-findings#81
• _depositToAave always returns 0 code-423n4/2021-06-pooltogether-findings#80
• Missing calls to init functions of inherited contracts code-423n4/2021-06-pooltogether-findings#60
• function _getRefferalCode() can be refactored to a constant variable code-423n4/2021-06-pooltogether-findings#20

[coveralls]

Coverage increased (+1.4%) to 100.0% when pulling 57ca88f on fixes/c4-audit into dc061c0 on main.