exclude vesting contracts from staking reward

currently, staking rewards are low, because fees are distributed according to the voting power of all stakers, including vesting contracts. This should be changed. The plan is to only distribute fees to voluntary stakers.

Currently, the staking contract is not differentiating between the 2 types of stakers (vesting vs voluntary), therefore we need to adjust the staking contract.

Option 1:

• Additional checkpoints for weighted stake / voting power of vesting contracts
• In fee sharing proxy, subtract that number from total weighted stake / voting power
• In fee sharing check if sender is vesting contract
  -> higher costs for vesting, but does not affect regular stakers in any way
  -> fee sharing a bit more expensive

Option 2:

• 2 seperate balances for weighted stake / voting power: stakers and vesting contracts
• Sum them up on total weighted stake / voting power computation
• Fee sharing proxy reads only the stakers weighted stakes
  -> same costs for vesting + staking
  -> higher costs for all operations which read from the staking contract (voting + fee sharing)

Since the contracts were designed to keep the reading inexpensive, it is probably better to go with option 1, but this is up for discussion.