Add experimental s3backer mounter

This also adds some generic handling of stale umounts.
Fuse returns immediately and does not indicate that
the mounter has finished writing to the backend.
The process finding is sort of hacky as I could not
find a better way to get to the PID from a fuse mount.

pkg/s3/s3-driver_suite_test.go

@@ -113,4 +113,35 @@ var _ = Describe("S3Driver", func() {
 		})
 	})
 
+	Context("s3backer", func() {
+		socket := "/tmp/csi-s3backer.sock"
+		csiEndpoint := "unix://" + socket
+
+		cfg := &s3.Config{
+			AccessKeyID:     "FJDSJ",
+			SecretAccessKey: "DSG643HGDS",
+			Endpoint:        "http://127.0.0.1:9000",
+			Mounter:         "s3backer",
+		}
+		if err := os.Remove(socket); err != nil && !os.IsNotExist(err) {
+			Expect(err).NotTo(HaveOccurred())
+		}
+		driver, err := s3.NewS3("test-node", csiEndpoint, cfg)
+		if err != nil {
+			log.Fatal(err)
+		}
+		go driver.Run()
+
+		defer os.RemoveAll(mntDir)
+
+		Describe("CSI sanity", func() {
+			sanityCfg := &sanity.Config{
+				TargetPath:     mntDir,
+				Address:        csiEndpoint,
+				TestVolumeSize: 1,
+			}
+			sanity.GinkgoTest(sanityCfg)
+		})
+	})
+
 })