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/interface.go

@@ -1,10 +1,10 @@
 package batch
 
-import "github.com/lightningnetwork/lnd/kvdb"
+import "context"
 
 // Request defines an operation that can be batched into a single bbolt
 // transaction.
-type Request struct {
+type Request[Q any] struct {
 	// Opts holds various configuration options for a scheduled request.
 	Opts *SchedulerOptions
 
@@ -19,7 +19,7 @@ type Request struct {
 	// Update is applied alongside other operations in the batch.
 	//
 	// NOTE: This method MUST NOT acquire any mutexes.
-	Update func(tx kvdb.RwTx) error
+	Update func(tx Q) error
 
 	// OnCommit is called if the batch or a subset of the batch including
 	// this request all succeeded without failure. The passed error should
@@ -32,16 +32,16 @@ type Request struct {
 
 // SchedulerOptions holds various configuration options for a scheduled request.
 type SchedulerOptions struct {
-	// lazy should be true if we don't have to immediately execute this
+	// Lazy should be true if we don't have to immediately execute this
 	// request when it comes in. This means that it can be scheduled later,
 	// allowing larger batches.
-	lazy bool
+	Lazy bool
 }
 
 // NewDefaultSchedulerOpts returns a new SchedulerOptions with default values.
 func NewDefaultSchedulerOpts() *SchedulerOptions {
 	return &SchedulerOptions{
-		lazy: false,
+		Lazy: false,
 	}
 }
 
@@ -62,20 +62,18 @@ type SchedulerOption func(*SchedulerOptions)
 
 // LazyAdd will make the request be executed lazily, added to the next batch to
 // reduce db contention.
-//
-// NOTE: This is currently a no-op for any DB backend other than bbolt.
 func LazyAdd() SchedulerOption {
 	return func(opts *SchedulerOptions) {
-		opts.lazy = true
+		opts.Lazy = true
 	}
 }
 
 // Scheduler abstracts a generic batching engine that accumulates an incoming
 // set of Requests, executes them, and returns the error from the operation.
-type Scheduler interface {
+type Scheduler[Q any] interface {
 	// Execute schedules a Request for execution with the next available
 	// batch. This method blocks until the underlying closure has been
 	// run against the database. The resulting error is returned to the
 	// caller.
-	Execute(req *Request) error
+	Execute(ctx context.Context, req *Request[Q]) error
 }