Use a flag to increase or decrease the index of the pool instead of a heuristic

Description

AmmRouter:_shitPool() increases the index if the price is within 100 of the upper bound of the current pool price.

This is error prone as a fill could for example increase the price of a pool, but not be close enough to the upper bound, making the index decrease, entering in a wrong state.

Recommendation

Use the direction flag to increase or decrease the index, as in AmmRouter:_getQuoteValue().

Status

Addressed in #68c722c