Update algorithm to remove need for loops

[LayneHaber]

Fixes c4 issue numbers:

• 27
• 24

[sanchaymittal]

A bit unclear why are we using three mappings to track the active block number.

Can we do this:

  /// @dev Number of transactions in the block for that user.
  /// user -> block -> number of prepare transactions in a single block
 mapping(address => mapping(uint256 => uint256))  public activeTransactionBlocks;

function prepare(){
...
 activeTransactionBlocks[user][block.number] += 1;
...
}

// removing active blocks
require(activeTransactionBlocks[user][preparedBlock] > 0, "removeUserActiveBlocks: NONE");
activeTransactionBlocks[user][preparedBlock] -= 1;