Fix a missing getNewNursery(), and related cleanup

Summary:
When we use nursery chunks with +RTS -n<size>, when the current nursery
runs out we have to check whether there's another chunk available with
getNewNursery().  There was one place we weren't doing this: the ad-hoc
heap check in scheduleProcessInbox().

The impact of the bug was that we would GC too early when using nursery
chunks, especially in programs that used messages (throwTo between
capabilities could do this, also hs_try_putmvar()).

Test Plan: validate, also local testing in our application

Reviewers: bgamari, niteria, austin, erikd

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3749