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/migration13/migration_test.go

@@ -3,7 +3,7 @@ package migration13
 import (
 	"testing"
 
-	"github.com/coreos/bbolt"
+	"github.com/lightningnetwork/lnd/channeldb/kvdb"
 	"github.com/lightningnetwork/lnd/channeldb/migtest"
 )
 
@@ -111,10 +111,10 @@ func TestMigrateMpp(t *testing.T) {
 
 	migtest.ApplyMigration(
 		t,
-		func(tx *bbolt.Tx) error {
+		func(tx kvdb.RwTx) error {
 			return migtest.RestoreDB(tx, paymentsRootBucket, pre)
 		},
-		func(tx *bbolt.Tx) error {
+		func(tx kvdb.RwTx) error {
 			return migtest.VerifyDB(tx, paymentsRootBucket, post)
 		},
 		MigrateMPP,