Fast path estimate doesn't account for buffer and xERC20s

[just-a-node]

Background

Fast path estimates should currently account for a buffer of current available router liquidity. Right now that is not the case for some paths/assets.

Also, xERC20s need to check router liquidity as wlel.

Definition Of Done

UI should show slow path time estimate if transfer amount > 70% of total available router liquidity. Additionally, only the top 4 routers by available liquidity for a specific asset should be considered in the total available liquidity of this estimate.

Tasks

• Identify whether the buffer rule is applied and fix if not
• Identify if only top 4 routers is being considered and fix if not
• Identify if xERC20s are handling available router liquidity

[just-a-node]

A more efficient method to use in the UI for determining fast path: #6018