feat: posted order lru cache

[mfw78]

Problem

As there is no method available for the conditional order to tell the watch-tower what action to take when the order is already in the order book, this may result in continually polling each block / spamming the API endpoint.

Suggested solution

Until cowprotocol/composable-cow#74 is complete, we can at least save on spamming the order book by maintaining an LRU cache of orderUids that have been submitted. If we poll and get a duplicate orderUid that's sitting in the LRU cache, we simply don't post it.

Alternatives considered

A full implementation of cowprotocol/composable-cow#74, however there is likely some other underlying work that would be done around the polling to unify/refactor the interfaces. This would be implemented at the ComposableCoW, and not the smart order as such (to maintain the single entry-point multiplexing), and therefore is for a later revision.

Acceptance criteria

• Implemented an LRU cache to reduce pressure on the order book.