partially revert "window_size: explicit adjustments only"

This partially reverts commit 03ca902
in order to fix extreme slowdown when uploading to localhost via SFTP.

I was able to repeat the issue on RHEL-7 on localhost only.  It did not
occur when uploading via network and it did not occur on a RHEL-6 box
with the same version of libssh2.

The problem was that sftp_read() used a read-ahead logic to figure out
the window_size, but sftp_packet_read() called indirectly from
sftp_write() did not use any read-ahead logic.

src/channel.c

@@ -1760,6 +1760,15 @@ ssize_t _libssh2_channel_read(LIBSSH2_CHANNEL *channel, int stream_id,
         channel->read_state = libssh2_NB_state_created;
     }
 
+    /*
+     * =============================== NOTE ===============================
+     * I know this is very ugly and not a really good use of "goto", but
+     * this case statement would be even uglier to do it any other way
+     */
+    if (channel->read_state == libssh2_NB_state_jump1) {
+        goto channel_read_window_adjust;
+    }
+
     rc = 1; /* set to >0 to let the while loop start */
 
     /* Process all pending incoming packets in all states in order to "even
@@ -1868,6 +1877,27 @@ ssize_t _libssh2_channel_read(LIBSSH2_CHANNEL *channel, int stream_id,
            more off the network again */
         channel->read_state = libssh2_NB_state_created;
 
+    if(channel->remote.window_size < (LIBSSH2_CHANNEL_WINDOW_DEFAULT*30)) {
+        /* the window is getting too narrow, expand it! */
+
+      channel_read_window_adjust:
+        channel->read_state = libssh2_NB_state_jump1;
+        /* the actual window adjusting may not finish so we need to deal with
+           this special state here */
+        rc = _libssh2_channel_receive_window_adjust(channel,
+                                                    (LIBSSH2_CHANNEL_WINDOW_DEFAULT*60),
+                                                    0, NULL);
+        if (rc)
+            return rc;
+
+        _libssh2_debug(session, LIBSSH2_TRACE_CONN,
+                       "channel_read() filled %d adjusted %d",
+                       bytes_read, buflen);
+        /* continue in 'created' state to drain the already read packages
+           first before starting to empty the socket further */
+        channel->read_state = libssh2_NB_state_created;
+    }
+
     return bytes_read;
 }