Currently, if BroadcastTransaction() is called to rebroadcast a
transaction (e.g. by ResendWalletTransactions()), then we add the
transaction to the unbroadcast set. That transaction has already been
broadcast in the past, so peers are unlikely to request it again,
meaning RemoveUnbroadcastTx() won't be called and it won't be removed
from m_unbroadcast_txids.

Net processing will therefore continue to attempt rebroadcast for the
transaction every 10-15 minutes. This will most likely continue until
the node connects to a new peer which hasn't yet seen the transaction
(or perhaps indefinitely).

Fix by only adding the transaction to the broadcast set when it's added to the mempool.

ninja all check-all