Combined fixes

[pash7ka]

Fixes included

• Fix SavingsModule bugs which made hack possible
• Fixes inspired by MixBytes audit
• Fix high gas issue with rewards

Contracts changed (exluding tests)

• contracts/interfaces/access/IAccessModule.sol
• contracts/modules/access/AccessChecker.sol
• contracts/modules/access/AccessModule.sol
• contracts/modules/reward/RewardVestingModule.sol
• contracts/modules/savings/RewardDistributions.sol
• contracts/modules/savings/SavingsModule.sol
• contracts/modules/staking/StakingPool.sol
• contracts/modules/staking/StakingPoolBase.sol