multi: export channel status field in migration25

Previously, in `migration25.OpenChannel`, there was a private field
`chanStatus` used to keep track of the channel status. The following
migrations, `migration26` and `migration27` also have their own
`OpenChannel` defined, with `migration26` inherited from `migration25`,
and `migration27` inherited from `migration26`. The private field
`chanStatus`, however, is NOT inherited and each of the migrations uses
its own. This is fine for reading and writing as, under the hood, the
`chanStatus` is just a `uint8` value. Because each migration has its own
fetcher and putter, it can safely access its private field to read and
write it correctly.

The issue pops up when we use the method
`migration25.FundingTxPresent()`. Because it's evaluating its channel
status using its own private field `chanStatus`, this field would always
be the default value(`ChanStatusDefault`), leading the statement
`!c.hasChanStatus(ChanStatusRestored)` to always be true. Thus a
restored channel will be mistakenly considered to have funding tx
present, causing failures in reading the channel info in the following
migrations.

We fix this by exporting the `ChanStatus` field so its value can be set
by following migrations.

channeldb/migration26/channel.go

@@ -70,10 +70,6 @@ var (
 // NOTE: doesn't have the Packager field as it's not used in current migration.
 type OpenChannel struct {
 	mig25.OpenChannel
-
-	// chanStatus is the current status of this channel. If it is not in
-	// the state Default, it should not be used for forwarding payments.
-	chanStatus mig25.ChannelStatus
 }
 
 // FetchChanInfo deserializes the channel info based on the legacy boolean.
@@ -104,7 +100,7 @@ func FetchChanInfo(chanBucket kvdb.RBucket, c *OpenChannel, legacy bool) error {
 	}
 
 	c.ChanType = mig25.ChannelType(chanType)
-	c.chanStatus = mig25.ChannelStatus(chanStatus)
+	c.ChanStatus = mig25.ChannelStatus(chanStatus)
 
 	// If this is the legacy format, we need to read the extra two new
 	// fields.
@@ -239,7 +235,7 @@ func PutChanInfo(chanBucket kvdb.RwBucket, c *OpenChannel, legacy bool) error {
 	if err := mig.WriteElements(&w,
 		mig.ChannelType(c.ChanType), c.ChainHash, c.FundingOutpoint,
 		c.ShortChannelID, c.IsPending, c.IsInitiator,
-		mig.ChannelStatus(c.chanStatus), c.FundingBroadcastHeight,
+		mig.ChannelStatus(c.ChanStatus), c.FundingBroadcastHeight,
 		c.NumConfsRequired, c.ChannelFlags,
 		c.IdentityPub, c.Capacity, c.TotalMSatSent,
 		c.TotalMSatReceived,