Made the counted_t move operators be noexcept.
Made the movable_t and scoped pointer move operations be noexcept.

This is just so that somebody double-checking that they will not
throw will immediately see such.
Removed THROWS_NOTHING decls from clone_ptr_t.
Made clone_ptr_t move operations use noexcept.
Made home_thread_mixin_t move constructor use noexcept.
Added some comments about noexcept specifiers in clone_ptr_t and scoped pointers.
Changed comments to appease Tim.
Merge branch 'sam_noexcept' into next
Merge branch 'next' of github.com:rethinkdb/rethinkdb into next