doc: vm: update github issues link
zlib: allow zero values for level and strategy

This is a back-port of commit c9644fb from the master branch.
zlib: level can be negative

This is a back-port of commit e945903 from the master branch.
http: use an unref'd timer to fix delay in exit

Fixes #5763.
http: test for exit delay after request/response

Regression test for an exit delay caused by an unref'd timer in
http.js.