Set default window size to 2MB

The default channel window size used until now was 256KB. This value is
too small and results on a bottleneck on real-life networks where
round-trip delays can easily reach 300ms.

The issue was not visible because the configured channel window size
was being ignored and a hard-coded value of ~22MB being used instead,
but that was fixed on a previous commit.

This patch just changes the default window size
(LIBSSH2_CHANNEL_WINDOW_DEFAULT) to 2MB. It is the same value used by
OpenSSH and in our opinion represents a good compromise between memory
used and transfer speed.

Performance tests were run to determine the optimum value. The details
and related discussion are available from the following thread on the
libssh2 mailing-list:

http://www.libssh2.org/mail/libssh2-devel-archive-2013-10/0018.shtml
http://article.gmane.org/gmane.network.ssh.libssh2.devel/6543

An excerpt follows:

"I have been running some transfer test and measuring their speed.

My setup was composed of a quad-core Linux machine running Ubuntu 13.10
x86_64 with a LXC container inside. The data transfers were performed
from the container to the host (never crossing through a physical
network device).

Network delays were simulated using the tc tool. And ping was used to
verify that they worked as intended during the tests.

The operation performed was the equivalent to the following ssh command:

  $ ssh container "dd bs=16K count=8K if=/dev/zero" >/dev/null

Though, establishment and closing of the SSH connection was excluded
from the timings.

I run the tests several times transferring files of sizes up to 128MB
and the results were consistent between runs.

The results corresponding to the 128MB transfer are available here:

https://docs.google.com/spreadsheet/ccc?key=0Ao1yRmX6PQQzdG5wSFlrZl9HRWNET3ZyN0hnaGo5ZFE&usp=sharing

It clearly shows that 256KB is too small as the default window size.
Moving to a 512MB generates a great improvement and after the 1MB mark
the returns rapidly diminish. Other factors (TCP window size, probably)
become more limiting than the channel window size

For comparison I also performed the same transfers using OpenSSH. Its
speed is usually on par with that of libssh2 using a window size of 1MB
(even if it uses a 2MB window, maybe it is less aggressive sending the
window adjust msgs)."

Signed-off-by: Salvador Fandino <sfandino@yahoo.com>

include/libssh2.h

@@ -593,7 +593,7 @@ LIBSSH2_API int libssh2_poll(LIBSSH2_POLLFD *fds, unsigned int nfds,
                              long timeout);
 
 /* Channel API */
-#define LIBSSH2_CHANNEL_WINDOW_DEFAULT  (256*1024)
+#define LIBSSH2_CHANNEL_WINDOW_DEFAULT  (2*1024*1024)
 #define LIBSSH2_CHANNEL_PACKET_DEFAULT  32768
 #define LIBSSH2_CHANNEL_MINADJUST       1024