update pkg/sftp to fix requests accumulation

Include this patch:

pkg/sftp#393

to avoid request accumulation (no underlying fd) if we return an error.
Before this patch the accumulated requests are released only when the
client disconnects.

We use our fork for now to include

pkg/sftp#392

too

go.mod

@@ -57,7 +57,7 @@ require (
 
 replace (
 	github.com/jlaffaye/ftp => github.com/drakkan/ftp v0.0.0-20201114075148-9b9adce499a9
-	github.com/pkg/sftp => github.com/drakkan/sftp v0.0.0-20201105171632-4ab2ef794ae3
+	github.com/pkg/sftp => github.com/drakkan/sftp v0.0.0-20201116163812-863783342b7c
 	golang.org/x/crypto => github.com/drakkan/crypto v0.0.0-20201114074711-d051624c4fd2
 	golang.org/x/net => github.com/drakkan/net v0.0.0-20201114074615-8a2467084c77
 )

go.sum

@@ -121,8 +121,8 @@ github.com/drakkan/ftp v0.0.0-20201114075148-9b9adce499a9 h1:LPH1dEblAOO/LoG7yHP
 github.com/drakkan/ftp v0.0.0-20201114075148-9b9adce499a9/go.mod h1:2lmrmq866uF2tnje75wQHzmPXhmSWUt7Gyx2vgK1RCU=
 github.com/drakkan/net v0.0.0-20201114074615-8a2467084c77 h1:keiJPG0lodjq5Ep9XuDKGRQRC52pp/8NB8/1xbqmw+Y=
 github.com/drakkan/net v0.0.0-20201114074615-8a2467084c77/go.mod h1:sp8m0HH+o8qH0wwXwYZr8TS3Oi6o0r6Gce1SSxlDquU=
-github.com/drakkan/sftp v0.0.0-20201105171632-4ab2ef794ae3 h1:9rug3vpr/986h/KeITYQQx78DErOAkOes+ktVRyTPQQ=
-github.com/drakkan/sftp v0.0.0-20201105171632-4ab2ef794ae3/go.mod h1:fUqqXB5vEgVCZ131L+9say31RAri6aF6KDViawhxKK8=
+github.com/drakkan/sftp v0.0.0-20201116163812-863783342b7c h1:1pl6n1QuqucEf9fPTjkGMvwThWG/I5HOwc5+/7YuLjQ=
+github.com/drakkan/sftp v0.0.0-20201116163812-863783342b7c/go.mod h1:fUqqXB5vEgVCZ131L+9say31RAri6aF6KDViawhxKK8=
 github.com/dustin/go-humanize v0.0.0-20171111073723-bb3d318650d4/go.mod h1:HtrtbFcZ19U5GC7JDqmcUSB87Iq5E25KnS6fMYU6eOk=
 github.com/eapache/go-resiliency v1.1.0/go.mod h1:kFI+JgMyC7bLPUVY133qvEBtVayf5mFgVsvEsIPBvNs=
 github.com/eapache/go-xerial-snappy v0.0.0-20180814174437-776d5712da21/go.mod h1:+020luEh2TKB4/GOp8oxxtq0Daoen/Cii55CzbTV6DU=