Speed up restic init over slow SFTP links

pkg/sftp.Client.MkdirAll(d) does a Stat to determine if d exists and is
a directory, then a recursive call to create the parent, so the calls
for data/?? each take three round trips. Doing a Mkdir first should
eliminate two round trips for 255/256 data directories as well as all
but one of the top-level directories.

Also, we can do all of the calls concurrently. This may reintroduce some
of the Stat calls when multiple goroutines try to create the same
parent, but at the default number of connections, that should not be
much of a problem.

go.mod

@@ -38,7 +38,7 @@ require (
 	golang.org/x/crypto v0.0.0-20220321153916-2c7772ba3064
 	golang.org/x/net v0.0.0-20220325170049-de3da57026de
 	golang.org/x/oauth2 v0.0.0-20220309155454-6242fa91716a
-	golang.org/x/sync v0.0.0-20210220032951-036812b2e83c
+	golang.org/x/sync v0.0.0-20220722155255-886fb9371eb4
 	golang.org/x/sys v0.0.0-20220325203850-36772127a21f
 	golang.org/x/term v0.0.0-20210927222741-03fcf44c2211
 	golang.org/x/text v0.3.7