Mempool unbroadcast set

The mempool contains an "unbroadcast" set called m_unbroadcast_txids. As the name implies this stores the txids of transactions which are in our mempool but have not been verified as broadcast to the wider P2P network. This might occur for example if a transaction is submitted locally (e.g. from the wallet or RPC), but we are not yet connected to any peers.

1. When a transaction is submitted to the network via BroadcastTransaction() it is added to the unbroadcast set in the mempool, before PeerManager calls RelayTransaction() to attempt initial broadcast.

2. When a transaction is heard about from the P2P network (via getdata in response to an INV), the transaction will be removed from the unbroadcast set.

   Transactions are also removed from the set on reorgs, new blocks arriving or if they’ve "expired" via RemoveStaged()

PeerManager schedules ReattemptInitialBroadcast() to be run every 10 minutes. This function loops over the unbroadcast set and either attempts rebroadcast or removes the transaction from the unbroadcast set if it is no longer in our mempool.

amiti wrote a gist on her proposal to improve rebroadcast logic in Bitcoin Core.