Swaps Buffer Support

[johngrantuk]

Not 100% this is direction yet but likely.

New parameter added to swap steps. Basically the flag says "hey, the pool is not a pool, it's actually just a buffer".

For the batch router, each swap step has an extra boolean if the "pool" you're trading against is not a pool, but a wrapper, it has to be true in other words, for bb-a-usd, you'll have 3 steps:

• pool: waDAI, trade dai for waDAI. isBuffer = true
• pool: bb-a-usd, trade waDAI for waUSDC. isBuffer = false
• pool waUSDC, trade waUSDC for USDC. isBuffer = true

Preferably the API can indicate if a pool is a buffer or not. Open an issue when confirmed this approach.
Also need to update docs.

[johngrantuk]

Even using deploy6 there is still noticeable discrepancy between query and swap results. Joao is investigating. Apart from that the current draft PR is up to date and just has failing tests due to this issue.