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/kvdb.go

@@ -0,0 +1,41 @@
+package batch
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/lightningnetwork/lnd/kvdb"
+	"github.com/lightningnetwork/lnd/sqldb"
+)
+
+// BoltBatcher is a bbolt implementation of the sqldb.BatchedTx interface.
+type BoltBatcher[Q any] struct {
+	db kvdb.Backend
+}
+
+// NewBoltBackend creates a new BoltBackend instance.
+func NewBoltBackend[Q any](db kvdb.Backend) *BoltBatcher[Q] {
+	return &BoltBatcher[Q]{db: db}
+}
+
+// ExecTx will execute the passed txBody, operating upon generic
+// parameter Q (usually a storage interface) in a single transaction.
+//
+// NOTE: This is part of the sqldb.BatchedTx interface.
+func (t *BoltBatcher[Q]) ExecTx(_ context.Context, opts sqldb.TxOptions,
+	txBody func(Q) error, reset func()) error {
+
+	if opts.ReadOnly() {
+		return fmt.Errorf("read-only transactions not supported")
+	}
+
+	return kvdb.Update(t.db, func(tx kvdb.RwTx) error {
+		q, ok := any(tx).(Q)
+		if !ok {
+			return fmt.Errorf("unable to cast tx(%T) into the "+
+				"type expected by the BoltBatcher(%T)", tx, t)
+		}
+
+		return txBody(q)
+	}, reset)
+}