Disable writes sync in persistent cache (elastic#21754)

With default configuration, every write is synced to disk. For the
persistent cache, Beats not need to guarantee that everything is on disk,
as the data can be recovered from the source endpoints, but the database
was not being properly closed, so there could happen that most of the
recent data was lost if the beat is stopped.
Now, the processors can close their resources, so the database is
properly closed in an normal shutdown, so we can go on without syncs on
writes, what gives a much better performance.

(cherry picked from commit 85b95ee)

x-pack/libbeat/persistentcache/store.go

@@ -36,10 +36,7 @@ func newStore(logger *logp.Logger, dir, name string) (*Store, error) {
 	// Opinionated options for the use of badger as a store for metadata caches in Beats.
 	options := badger.DefaultOptions(dbPath)
 	options.Logger = badgerLogger{logger.Named("badger")}
-	// TODO: Disabling sync writes gives better performance, and data loss wouldn't
-	// be a problem for caches. But we are not properly closing processors yet, so let
-	// sync on writes by now.
-	// options.SyncWrites = false
+	options.SyncWrites = false
 
 	db, err := badger.Open(options)
 	if err != nil {