channeldb: convert to uniformly use new kvdb abstractions

In this commit, we migrate all the code in `channeldb` to only reference
the new `kvdb` package rather than `bbolt` directly.

In many instances, we need to add two version to fetch a bucket as both
read and write when needed. As an example, we add a new
`fetchChanBucketRw` function. This function is identical to
`fetchChanBucket`, but it will be used to fetch the main channel bucket
for all _write_ transactions. We need a new method as you can pass a
write transaction where a read is accepted, but not the other way around
due to the stronger typing of the new `kvdb` package.

channeldb/migration12/migration.go

@@ -3,7 +3,7 @@ package migration12
 import (
 	"bytes"
 
-	"github.com/coreos/bbolt"
+	"github.com/lightningnetwork/lnd/channeldb/kvdb"
 	"github.com/lightningnetwork/lnd/lnwire"
 )
 
@@ -12,11 +12,11 @@ var emptyFeatures = lnwire.NewFeatureVector(nil, nil)
 // MigrateInvoiceTLV migrates all existing invoice bodies over to be serialized
 // in a single TLV stream. In the process, we drop the Receipt field and add
 // PaymentAddr and Features to the invoice Terms.
-func MigrateInvoiceTLV(tx *bbolt.Tx) error {
+func MigrateInvoiceTLV(tx kvdb.RwTx) error {
 	log.Infof("Migrating invoice bodies to TLV, " +
 		"adding payment addresses and feature vectors.")
 
-	invoiceB := tx.Bucket(invoiceBucket)
+	invoiceB := tx.ReadWriteBucket(invoiceBucket)
 	if invoiceB == nil {
 		return nil
 	}