## Linked Issues/PRs
Part of #1049

## Description
This PR introduce a new workflow when we successfully subscribed to the
TX topic in gossip we try ask for all the tx_ids in the pool's peer and
then ask the full transactions that we don't have.
This PR also build the `SharedState` of `P2PService` independently to
give more flexibility to the initialisation and avoid cyclic
dependencies.

### Detailed workflow : 

- When a peer has subscribed to our topic send inform the TxPool
- The TxPool asks the tx ids of the peer
- The TxPool filter the one it already knows and ask for the full
transactions of the others (a multi message send should be done if the
number of txs asked is above the limit (currently 10k chosen randomly)).
- We verify and add them to our txpool

### Questions for review

- What's happens if the peer doesn't answer to a request that we await ?
- I placed Option in the request/response because there was in the other
messages but why it's needed ?

## Checklist
- [x] Breaking changes are clearly marked as such in the PR description
and changelog
- [x] New behavior is reflected in tests
- [x] [The specification](https://github.com/FuelLabs/fuel-specs/)
matches the implemented behavior (link update PR if changes are needed)

### Before requesting review
- [x] I have reviewed the code myself
- [x] I have created follow-up issues caused by this PR and linked them
here

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Co-authored-by: Green Baneling <XgreenX9999@gmail.com>