MAX_SFTP_OUTGOING_SIZE: 30000

I ran SFTP upload tests against localhost. It showed that to make the
app reach really good speeds, I needed to do a little code tweak and
change MAX_SFTP_OUTGOING_SIZE from 4000 to 30000. The tests I did before
with the high latency tests didn't show any real difference whatever I
had that size set to.

This number is the size in bytes that libssh2 cuts off the large input
buffer and sends off as an individual sftp packet.

src/sftp.h

@@ -42,7 +42,7 @@
 /*
  * MAX_SFTP_OUTGOING_SIZE MUST not be larger than 32500 or so
  */
-#define MAX_SFTP_OUTGOING_SIZE 4000
+#define MAX_SFTP_OUTGOING_SIZE 30000
 
 struct sftp_write_chunk {
     struct list_node node;