Simplify the migration handover protocol

The three-way hand-off has a problem: there's no way to arrange for the
state of the migration to be unambiguous in case of failure.  If the
final "disconnect" message is lost (as in, the destination never
receives it whether it is sent by the sender or not), the destination
has no option but to quit with an error status and let a human sort it
out.  However, at that point we can either arrange to have a .INCOMPLETE
file still on disc or not - and it doesn't matter which we choose, we
can still end up with dataloss by picking a specific calamity to have
befallen the sender.

Given this, it makes sense to fall back to a simpler protocol: just send
all the data, then send a "disconnect" message.  This has the same
downside that we need a human to sort out specific failure cases, but
combined with --unlink before sending "disconnect" (see next patch) it
will always be possible for a human to disambiguate, whether the
destination quit with an error status or not.

tests/acceptance/test_happy_path.rb

@@ -72,13 +72,14 @@ class TestHappyPath < Test::Unit::TestCase
     @env.listen2
 
     @env.nbd1.can_die
+    @env.nbd2.can_die(0)
     stdout, stderr = @env.mirror12
 
     @env.nbd1.join
+    @env.nbd2.join
 
     assert_equal(@env.file1.read_original( 0, @env.blocksize ),
                  @env.file2.read( 0, @env.blocksize ) )
-    assert @env.status2['has_control'], "destination didn't take control"
   end