sftpfs: add buffering support

this way we improve performance over high latency networks

httpd/schema/openapi.yaml

@@ -1613,6 +1613,9 @@ components:
         disable_concurrent_reads:
           type: boolean
           description: Concurrent reads are safe to use and disabling them will degrade performance. Some servers automatically delete files once they are downloaded. Using concurrent reads is problematic with such servers.
+        buffer_size:
+          type: intger
+          description: The size of the buffer (in MB) to use for transfers. By enabling buffering, the reads and writes, from/to the remote SFTP server, are split in multiple concurrent requests and this allows data to be transferred at a faster rate, over high latency networks, by overlapping round-trip times. With buffering enabled, resuming uploads is not supported and a file cannot be opened for both reading and writing at the same time. 0 means disabled.
     FilesystemConfig:
       type: object
       properties: