Made shared_connection_block default constructible. Split shared_connection_block

tests out of deletion_test, and added a little test for a 
default-constructed shared_connection_block.


[SVN r53128]

libs/signals2

@@ -1 +1 @@
-Subproject commit 87ff0db5fcc965d0478d668bd24910e07ad2a8ce
+Subproject commit ca91bb77bded2666363501fc24957848aa31cafd