Put back dtrace probes

[misterdjules]

dtrace probes have been removed recently because:

• They were causing build issues on some platforms.
• Only 2 probes were defined, which even on a platform where they would work properly, was not considered a sufficient use case.

However, given the usefulness of dtrace probes for troubleshooting issues in general and in node.js, my opinion is that we should investigate how to:

• Make these probes work properly on platforms where they do not.
• Identify other places in the code base where such probes would be useful.

[saghul]

I don't disagree :-) If you are willing to do the work please open an issue / PR at https://github.com/libuv/libuv so we can discuss the way forward.