Download WAL files in parallel during restore

This commit changes the restore to download multiple WAL files to
the local disk in parallel while another goroutine applies those
files in order. Downloading & applying the WAL files in serial
reduces the total throughput as WAL files are typically made up of
multiple small files.

cmd/litestream/restore.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"log"
 	"os"
+	"strconv"
 	"time"
 
 	"github.com/benbjohnson/litestream"
@@ -26,6 +27,7 @@ func (c *RestoreCommand) Run(ctx context.Context, args []string) (err error) {
 	fs.StringVar(&opt.ReplicaName, "replica", "", "replica name")
 	fs.StringVar(&opt.Generation, "generation", "", "generation name")
 	fs.Var((*indexVar)(&opt.Index), "index", "wal index")
+	fs.IntVar(&opt.Parallelism, "parallelism", opt.Parallelism, "parallelism")
 	ifReplicaExists := fs.Bool("if-replica-exists", false, "")
 	timestampStr := fs.String("timestamp", "", "timestamp")
 	verbose := fs.Bool("v", false, "verbose output")
@@ -170,6 +172,10 @@ Arguments:
 	-if-replica-exists
 	    Returns exit code of 0 if no backups found.
 
+	-parallelism NUM
+	    Determines the number of WAL files downloaded in parallel.
+	    Defaults to `+strconv.Itoa(litestream.DefaultRestoreParallelism)+`.
+
 	-v
 	    Verbose output.