Fix/doublespend in optimistic

[AllFi]

This PR more or less replicates the behavior of #226 but replaces optimistic indexes with timestamps.

The general idea is to maintain a cache of pending transactions. These pending transactions are merged with already indexed transactions in response to a /transactions/v2 request. The merge process is straightforward: it removes duplicates between the indexed and pending transactions based on commitment and adds the remaining pending transactions, sorted by timestamp, up to the specified limit.

There are three events that trigger the removal of pending transactions:

• The sent transaction fails.
• The transaction is indexed by the indexer.
• The transaction has been in the cache for too long (1 day), though ideally, this situation should never occur.

One consequence of this PR is that pendingDeltaIndex in the /info response is now less accurate. However, this shouldn't be an issue, as it is only used to estimate the limit value for the /transactions/v2 request.

[EvgenKor]

lgtm