Store pending transactions when shutting down agents

[Adamantios]

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Is your feature request related to a problem? Please describe. When using the transaction settlement skill, we might have pending transactions which are not cleared when we shut down the agents.

Describe the solution you'd like Store pending transactions when shutting down agents, so that we know which transactions are pending. When restarting the skill, we can use their hash to retrieve the prices set and nonce used (which we can also get from chain) and then resend with properly increased price.