batch+graph: update batch.Schedular to be generic

In preparation for using the same logic for non-bbolt backends, we adapt
the batch.Schedular to be more generic.

The only user of the scheduler at the moment is the KVStore in the
`graph.db` package. This store instantiates the bbolt implementation of
the scheduler.

batch/batch.go

@@ -1,10 +1,10 @@
 package batch
 
 import (
+	"context"
 	"errors"
 	"sync"
 
-	"github.com/lightningnetwork/lnd/kvdb"
 	"github.com/lightningnetwork/lnd/sqldb"
 )
 
@@ -14,28 +14,30 @@ var errSolo = errors.New(
 	"batch function returned an error and should be re-run solo",
 )
 
-type request struct {
-	*Request
+type request[Q any] struct {
+	*Request[Q]
 	errChan chan error
 }
 
-type batch struct {
-	db     kvdb.Backend
+type batch[Q any] struct {
+	db     sqldb.BatchedTx[Q]
 	start  sync.Once
-	reqs   []*request
-	clear  func(b *batch)
+	reqs   []*request[Q]
+	clear  func(b *batch[Q])
 	locker sync.Locker
 }
 
 // trigger is the entry point for the batch and ensures that run is started at
 // most once.
-func (b *batch) trigger() {
-	b.start.Do(b.run)
+func (b *batch[Q]) trigger(ctx context.Context) {
+	b.start.Do(func() {
+		b.run(ctx)
+	})
 }
 
 // run executes the current batch of requests. If any individual requests fail
 // alongside others they will be retried by the caller.
-func (b *batch) run() {
+func (b *batch[Q]) run(ctx context.Context) {
 	// Clear the batch from its scheduler, ensuring that no new requests are
 	// added to this batch.
 	b.clear(b)
@@ -50,9 +52,10 @@ func (b *batch) run() {
 
 	// Apply the batch until a subset succeeds or all of them fail. Requests
 	// that fail will be retried individually.
+	var writeTx writeOpts
 	for len(b.reqs) > 0 {
 		var failIdx = -1
-		err := kvdb.Update(b.db, func(tx kvdb.RwTx) error {
+		err := b.db.ExecTx(ctx, &writeTx, func(tx Q) error {
 			for i, req := range b.reqs {
 				err := req.Update(tx)
 				if err != nil {