Support parsing persisted v2 channel monitor names

ChannelMonitors are persisted using the corresponding channel's funding
outpoint for the key. This is fine for v1 channels since the funding
output will never change. However, this is not the case for v2 channels
since a splice will result in a new funding outpoint.

Support parsing a MonitorName for v2 channels as a ChannelId, which
still looks like a outpoint only without an index. This allows
differentiating monitors for v1 channels from those of v2 channels.

Author: jkczyz