Use shared_ptr::unique() instead of shared_ptr::use_count() == 1.

Use shared_ptr::unique() instead of shared_ptr::use_count() == 1.

use_count() can be slow (not constant time) depending on underlying implementation.
Similar situation to std::list::size() == 0 an std::list::empty(), the former
does a big traversal/computation only to check if the very first step went.
I think the current boost implementation is just a reference count anyway, but
hey.  In theory.

git-svn-id: svn://localhost/ardour2/branches/3.0@5996 d708f5d6-7413-0410-9779-e7cbd77b26cf