Explicitly track the set of spendable transactions which confirm

Explicitly track the set of spendable transactions which confirm

In `ChannelMonitor`s, when a transaction containing a spend of a
revoked remote output reaches 6 confs, we may have no other
tracking of that txid remaining. Thus, if we see that transaction
again (because a user duplicatively confirms it), we'll generate a
redundant spendable output event for it.

Here we simply explicitly track all txids of transactions which
confirm with a spendable output, allowing us to check this
condition in the next commit.