Merge #18038: P2P: Mempool tracks locally submitted transactions to improve wallet privacy

50fc4df6c4e8a84bdda13ade7bed7a2131796f00 [mempool] Persist unbroadcast set to mempool.dat (Amiti Uttarwar)
297a1785360c4db662a7f3d3ade7b6b503258d39 [test] Integration tests for unbroadcast functionality (Amiti Uttarwar)
6851502472d3625416f0e7796e9f2a0379d14d49 [refactor/test] Extract P2PTxInvStore into test framework (Amiti Uttarwar)
dc1da48dc5e5526215561311c184a8cbc345ecdc [wallet] Update the rebroadcast frequency to be ~1/day. (Amiti Uttarwar)
e25e42f20a3aa39651fbc1f9fa3df1a49f1f5868 [p2p] Reattempt initial send of unbroadcast transactions (Amiti Uttarwar)
7e93eecce3bc5a1b7bb0284e06f9e2e69454f5ba [util] Add method that returns random time in milliseconds (Amiti Uttarwar)
89eeb4a3335f8e871cc3f5286af4546dff66172a [mempool] Track "unbroadcast" transactions (Amiti Uttarwar)

Pull request description:

  This PR introduces mempool tracking of unbroadcast transactions and periodic reattempts at initial broadcast. This is a part of the rebroadcast project, and a standalone privacy win.

  The current rebroadcast logic is terrible for privacy because 1. only the source wallet rebroadcasts transactions and 2. it does so quite frequently. In the current system, if a user submits a transaction that does not immediately get broadcast to the network (eg. they are offline), this "rebroadcast" behavior is the safety net that can actually serve as the initial broadcast. So, keeping the attempts frequent is important for initial delivery within a reasonable timespan.

  This PR aims to improve # 2 by reducing the wallet rebroadcast frequency to ~1/day from ~1/15 min. It achieves this by separating the notion of initial broadcast from rebroadcasts. With these changes, the mempool tracks locally submitted transactions & periodically reattempts initial broadcast. Transactions submitted via the wallet or RPC are added to an "unbroadcast" set & are removed when a peer sends a `getdata` request, or the transaction is removed from the mempool. Every 10-15 minutes, the node reattempts an initial broadcast. This enables reducing the wallet rebroadcast frequency while ensuring the transactions will be propagated to the network.

  For privacy improvements around # 1, please see #16698.
  Thank you to gmaxwell for the idea of how to break out this subset of functionality (https://github.com/bitcoin/bitcoin/pull/16698#issuecomment-571399346)

ACKs for top commit:
  fjahr:
    Code review ACK 50fc4df6c4e8a84bdda13ade7bed7a2131796f00
  MarcoFalke:
    ACK 50fc4df6c4e8a84bdda13ade7bed7a2131796f00, I think this is ready for merge now 👻
  amitiuttarwar:
    The current tip `50fc4df` currently has 6 ACKs on it, so I've opened #18807 to address the last bits.
  jnewbery:
    utACK 50fc4df6c4e8a84bdda13ade7bed7a2131796f00.
  ariard:
    Code Review ACK 50fc4df (minor points no need to invalid other ACKs)
  robot-visions:
    ACK 50fc4df6c4e8a84bdda13ade7bed7a2131796f00
  sipa:
    utACK 50fc4df6c4e8a84bdda13ade7bed7a2131796f00
  naumenkogs:
    utACK 50fc4df

Tree-SHA512: 2dd935d645d5e209f8abf87bfaa3ef0e4492705ce7e89ea64279cb27ffd37f4727fa94ad62d41be331177332f8edbebf3c7f4972f8cda10dd951b80a28ab3c0f

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