src: fix StreamPipe cleanup & interaction with env handles

src: do not access Environment-owned handles after cleanup

Do not access handles that have already begun to be closed or are closed.

src: clean up StreamPipe in destructor

In the presence of Workers, it is not safe to assume that StreamPipe::Unpipe() has been called at the time when the object is destroyed.

Instead, clean up when the destructor is called.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines