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.

contractcourt/chain_watcher.go

@@ -269,7 +269,7 @@ func (c *chainWatcher) Start() error {
 	// at.
 	heightHint := c.cfg.chanState.ShortChanID().BlockHeight
 	if heightHint == 0 {
-		heightHint = chanState.FundingBroadcastHeight
+		heightHint = chanState.BroadcastHeight()
 	}
 
 	localKey := chanState.LocalChanCfg.MultiSigKey.PubKey.SerializeCompressed()