process: make getActive{Handles,Requests} official

[cjihrig]

process._getActiveHandles() and process._getActiveRequests() have been around for a long time, and are utilized to a large enough degree that it makes sense to make them official
APIs.

If this PR is accepted, I'll submit a follow up that deprecates the underscore prefixed versions.

Fixes: #1128

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

process