Fix bond handling during nodelet unloading

[dseifert]

This fixes points 2 and 3 of #50 and introduces two changes:

1. The callback for broken bonds is reset when unloading, to avoid calling unload() again.
2. nodelet load uses an ASyncSpinner in order to handle bond communication during shutdown.

[mikaelarguedas]

nice catch! tested locally and it works just fine, all nodelets are unloaded properly.
Would it be possible to provide an automated test with numerous nodelet loads and confirming that they are all unloaded properly?

Thanks for the contribution!

[dseifert]

What is the definition of a nodelet being unloaded properly?

[dseifert]

@mikaelarguedas I added tests for my changes. I don't think it is necessary to add a test for multiple nodelets for these, as the issues are reproducable with single nodelets, too (though "success" for the proper bond handling is a bit hard to check for right now, basically it is not getting a certain DEBUG output).

[mikaelarguedas]

@dseifert Thanks for adding the tests, multiple nodelets unloading was not tested before and this PR definitely improves of the unloading behaviour regardless of the number of nodelets. So I'm in favor of merging it.
Thanks for the contribution

[kejxu]

thanks for the detailed explanation @dseifert, really helpful! Just curious, why is single-threaded ros::AsyncSpinner spinner(1) needed? In other words, what is the problem with ros::spinOnce();? Thanks!