src: expose libuv handle/req id on process object

Assign numeric identifiers to libuv handle and request wrappers and
expose them to JS land as process._uvContextId[0].  Provides an ersatz
"thread-local" state for tracking execution across asynchronous calls.

Future optimization: reuse identifiers so they keep fitting in a SMI?

R=@trevnorris?

This PR is a conversation starter.  The goal is to have a cheap way of tracking the flow of execution across callbacks for instrumentation purposes.