channeldb: BigSize migration, store zero-conf, scid-alias bits

This introduces a BigSize migration that is used to expand the width
of the ChannelStatus and ChannelType fields. Three channel "types"
are added - ZeroConfBit, ScidAliasChanBit, and ScidAliasFeatureBit.
ScidAliasChanBit denotes that the scid-alias channel type was
negotiated for the channel. ScidAliasFeatureBit denotes that the
scid-alias feature bit was negotiated during the *lifetime* of the
channel. Several helper functions on the OpenChannel struct are
exposed to aid callers from different packages.

The RefreshShortChanID has been renamed to Refresh.

A new function BroadcastHeight is used to guard access to the
mutable FundingBroadcastHeight member. This prevents data races.

chanbackup/single.go

@@ -177,7 +177,7 @@ func NewSingle(channel *channeldb.OpenChannel,
 	// to the channel ID so we can use that as height hint on restore.
 	chanID := channel.ShortChanID()
 	if chanID.BlockHeight == 0 {
-		chanID.BlockHeight = channel.FundingBroadcastHeight
+		chanID.BlockHeight = channel.BroadcastHeight()
 	}
 
 	single := Single{