Retry failed rebind attempts

When we receive a migration, if rebinding to the new listen address and
port fails for a reason which might be fixable, rather than killing the
server we retry once a second.  Also in this patch: non-overlapping log
messages and a fix for the client going away halfway through a sendfile
loop.

tests/acceptance/fakes/source/close_mid_read.rb

@@ -0,0 +1,17 @@
+#!/usr/bin/env ruby
+
+# Connect, but get the protocol wrong: don't read the hello, so we
+# close and break the sendfile.
+
+require 'flexnbd/fake_source'
+include FlexNBD
+
+addr, port, srv_pid, newaddr, newport = *ARGV
+
+client = FakeSource.new( addr, port, "Timed out connecting" )
+client.write_read_request( 0, 8 )
+client.read_raw( 4 )
+client.close
+
+
+exit(0)