mitogen: Compress first stage without header or checksum (790 bytes -> 786)

```console
➜  mitogen git:(boot-cmd--argv) ✗ ./preamble_size.py
SSH command size: 786
Preamble (mitogen.core + econtext) size: 18230 (17.80KiB)

                        Original           Minimized           Compressed
mitogen.core         152237 148.7KiB  68453 66.8KiB 45.0%  18130 17.7KiB 11.9%
mitogen.parent        99166  96.8KiB  51375 50.2KiB 51.8%  12957 12.7KiB 13.1%
mitogen.fork           8445   8.2KiB   4139  4.0KiB 49.0%   1652  1.6KiB 19.6%
mitogen.ssh           10847  10.6KiB   6913  6.8KiB 63.7%   2102  2.1KiB 19.4%
mitogen.sudo          12089  11.8KiB   5924  5.8KiB 49.0%   2249  2.2KiB 18.6%
mitogen.select        12325  12.0KiB   2929  2.9KiB 23.8%    964  0.9KiB  7.8%
mitogen.service       41581  40.6KiB  22398 21.9KiB 53.9%   5847  5.7KiB 14.1%
mitogen.fakessh       15753  15.4KiB   8135  7.9KiB 51.6%   2672  2.6KiB 17.0%
mitogen.master        52891  51.7KiB  27586 26.9KiB 52.2%   7129  7.0KiB 13.5%
```

Confirmed Python 2.4 supports this use of zlib.compressobj, despite lack of
mention in https://docs.python.org/2.4/lib/module-zlib.html

```pycon
Python 2.4.6 (#2, Apr 29 2018, 11:16:24)
[GCC 7.3.0] on linux4
Type "help", "copyright", "credits" or "license" for more information.
>>> import zlib
>>> c=zlib.compressobj(zlib.Z_BEST_COMPRESSION,zlib.DEFLATED,-zlib.MAX_WBITS)
>>> c.compress('qwertyuiop') + c.flush()
'+,O-*\xa9,\xcd\xcc/\x00\x00'
```

mitogen/parent.py

@@ -1474,7 +1474,10 @@ class Connection(object):
         preamble_compressed = self.get_preamble()
         source = source.replace('PREAMBLE_COMPRESSED_LEN',
                                 str(len(preamble_compressed)))
-        compressed = zlib.compress(source.encode(), 9)
+        compressor = zlib.compressobj(
+            zlib.Z_BEST_COMPRESSION, zlib.DEFLATED, -zlib.MAX_WBITS,
+        )
+        compressed = compressor.compress(source.encode()) + compressor.flush()
         encoded = binascii.b2a_base64(compressed).replace(b('\n'), b(''))
 
         # Just enough to decode, decompress, and exec the first stage.
@@ -1485,7 +1488,7 @@ class Connection(object):
             '-c',
             'import sys;sys.path=[p for p in sys.path if p];'
             'import binascii,os,select,zlib;'
-            'exec(zlib.decompress(binascii.a2b_base64("%s")))' % (encoded.decode(),),
+            'exec(zlib.decompress(binascii.a2b_base64("%s",-15)))' % (encoded.decode(),),
         ]
 
     def get_econtext_config(self):