Lock outbound channels at 0conf if the peer indicates support for it

Lock outbound channels at 0conf if the peer indicates support for it

If our peer sets a minimum depth of 0, and we're set to trusting
ourselves to not double-spend our own funding transactions, send a
funding_locked message immediately after funding signed.

Note that some special care has to be taken around the
`channel_state` values - `ChannelFunded` no longer implies the
funding transaction is confirmed on-chain. Thus, for example, the
should-we-re-broadcast logic has to now accept `channel_state`
values greater than `ChannelFunded` as indicating we may still need
to re-broadcast our funding tranasction, unless `minimum_depth` is
greater than 0.

Further note that this starts writing `Channel` objects with a
`MIN_SERIALIZATION_VERSION` of 2. Thus, LDK versions prior to
0.0.99 (July 2021) will now refuse to read serialized
Channels/ChannelManagers.