Keep backingBuffer in BackingManager to pay for slippage during rebalances

[tmattimore]

The needed = needed.mul(FIX_ONE.plus(backingBuffer)); line needs to be above the RSR minting block if we want to keep capital around up to the BackingBuffer.

Or, alternatively, we can keep it where it is but not forward the RToken in the loop at the bottom up to the correct amount.

[tmattimore]

@tbrent is this a 3.0.0?