Limit the output following depth

## Status quo

`onchaind` adds a `txowatch` for the funding output of a channel when
the funding transaction is confirmed. This is so that `lightningd` can
inform `onchaind` and other channel-specific daemons about spends that
they are interested in.

The notification of spends is asynchronous, and so `lightningd`
implements an auto-follow logic: if an output we are watching is being
spent, then we add the spend transaction's outputs also to the set of
watch TX outputs. This allows for correct tracking despite `onchaind`
handling the notifications asynchronously.

This however also means that if `onchaind` is very slow, for example
because the signer is not present, and we're waiting on a signature,
then this adds new outputs to the watched set faster than we can
unsubscribe the not interesting ones.

We've seen at least one case in which a channel closing resulted in
50k UTXO spend notifications being queued up, and a similar number of
`txowatch` instances.

## Solution

Channels have a predictable max depth of interesting transactions for
`onchaind`. For example the longest chain that I can think of at the
moment is:

1. Funding output
2. HTLC second stage output
3. HTLC second stage sweep output
4. HTLC second stage sweep anchor spend

So if we add a `maxdepth` field to the `txowatch` we can count that
down every time we create a descendant `txowatch` from a parent, and
if it hits `0` we do not follow further. By setting `maxdepth` to a
large(-ish) number (say `10`) we still follow correctly, but no longer
risk picking up some mixer structure that fans our attention out to
thousands of outputs to follow.

## Impact

The sheer number of UTXO spends being replayed during startup was
causing `onchaind` to fall further and further behind, all the while
the front of observed UTXOs was increasing rapidly.