Lido pushBeacon with _beaconValidators amount for proper reward calculation

[ongrid]

To implement the new reward algorithm proposed in LIP-3, existing reportEther2 method needs to be re-implemented:

• The new interface: function pushBeacon(uint128 _beaconValidators, uint256 _beaconBalance) external... now receives the number of validators in beacon validator set
• Implement getInProcessBalance() method that eturns the total base balance (multiple of 32) of validators in transient state. The value nominated in wei (1e-18 Ether)
• New reward calculation, that treats validators that are in a transient state to be accounted
• The reward distribution will happen only in case of positive reward since previous report (rewardBase calculation principles to be changed). RewardBase will be calculated dynamically and not stored. Instead of this, Lido stores previously pushed beaconBalance - lastBeaconBalance.
• Lido contract becomes epoch-awareness, epochs are checked only in Oracle contract
• Remove all deprecated code
• Cosmetic: More clear naming for pushBeacon to avoid confusions with oracles reports
• Cosmetic: Eth2 and remote renamed to beacon to unify within codebase.
• Cosmetic: _getTotalControlledEther() will be renamed to getTotalPooledEth()
• Cosmetic: all introduced entities should have docstrings

Tests:

• Check transit states when validator becomes visible on beacon (illustrated in spec)
• Slashing/loss leads to decreasing stETH supply
• Appearing new validator from the in-fly state doesn't change beaconBalance (illustrated in spec)
• The reward after loss/slashing would be possible even the balance didn't recover to initial

Solves functional: #110, #4 and cosmetic: #87, #71