[stream-refactor] send MITO002 earlier

Prevents 2.4 bootstrap from attempting to fetch os_fork too early.

Connection(None).connect(): pid:25098 stdin:81 stdout:81 stderr:79
ssh.localhost:2201: (partial): mitogen__has_sudo_nopw@localhost's password:
ssh.localhost:2201: (password prompt): mitogen__has_sudo_nopw@localhost's password:
ssh.localhost:2201: (unrecognized): mitogen__has_sudo_nopw@localhost's password:
BootstrapProtocol(ssh.localhost:2201): first stage started succcessfully
BootstrapProtocol(ssh.localhost:2201): first stage received bootstrap
ssh.localhost:2201: (partial): MIdmitogen.os_fork
ssh.localhost:2201: (unrecognized partial): MIdmitogen.os_fork
ssh.localhost:2201: failing connection due to TimeoutError(u'Failed to setup connection after 10.00 seconds',)

mitogen/core.py

@@ -3599,9 +3599,9 @@ class ExternalContext(object):
                                  socket.gethostname())
                 _v and LOG.debug('Recovered sys.executable: %r', sys.executable)
 
-                self.broker._py24_25_compat()
                 if self.config.get('send_ec2', True):
                     self.stream.transmit_side.write(b('MITO002\n'))
+                self.broker._py24_25_compat()
                 self.log_handler.uncork()
                 self.dispatcher.run()
                 _v and LOG.debug('ExternalContext.main() normal exit')