ssh: enable compression by default

Using the same test as in 7af97c0365,
transmitted wire bytes drops from 135,531 to 133,071 (-1.81%), while
received drops from 21,073 to 14,775 (-30%).

Combined, both changes shave 13,914 bytes (-8.6%) off aggregate
bandwidth usage.

Make it configurable as compression hurts in some scenarios.

docs/api.rst

@@ -562,7 +562,7 @@ Router Class
             a password is requested, :py:class:`mitogen.sudo.PasswordError` is
             raised.
 
-    .. method:: ssh (hostname, username=None, ssh_path=None, port=None, check_host_keys=True, password=None, identity_file=None, \**kwargs)
+    .. method:: ssh (hostname, username=None, ssh_path=None, port=None, check_host_keys=True, password=None, identity_file=None, compression_level=6, \**kwargs)
 
         Arrange for a context to be constructed over a ``ssh`` invocation. The
         ``ssh`` process is started in a newly allocated pseudo-terminal, and
@@ -594,6 +594,10 @@ Router Class
             the SSH client to perform authenticaion; agent authentication is
             automatically disabled, as is reading the default private key from
             ``~/.ssh/id_rsa``, or ``~/.ssh/id_dsa``.
+        :param int compression_level:
+            Integer 0-9 representing the zlib compression level to use on the
+            connection, with 0 indicating compression is disabled. Defaults to
+            6.
 
 
 Context Class

mitogen/ssh.py

@@ -59,7 +59,7 @@ class Stream(mitogen.parent.Stream):
 
     def construct(self, hostname, username=None, ssh_path=None, port=None,
                   check_host_keys=True, password=None, identity_file=None,
-                  **kwargs):
+                  compression_level=6, **kwargs):
         super(Stream, self).construct(**kwargs)
         self.hostname = hostname
         self.username = username
@@ -67,6 +67,7 @@ class Stream(mitogen.parent.Stream):
         self.check_host_keys = check_host_keys
         self.password = password
         self.identity_file = identity_file
+        self.compression_level = compression_level
         if ssh_path:
             self.ssh_path = ssh_path
 
@@ -82,6 +83,11 @@ class Stream(mitogen.parent.Stream):
             bits += ['-o', 'IdentitiesOnly yes']
         if self.identity_file:
             bits += ['-i', self.identity_file]
+        if self.compression_level:
+            bits += [
+                '-o', 'Compression yes',
+                '-o', 'CompressionLevel %d' % (self.compression_level,)
+            ]
         if not self.check_host_keys:
             bits += [
                 '-o', 'StrictHostKeyChecking no',