[sgen] Make nursery collector for parallel M&S non-parallel again.

[sgen] Make nursery collector for parallel M&S non-parallel again.

Thread synchronization for the parallel nursery collector is (still)
too high to pay off.  Also, it currently crashes on the corlib
testsuite.

It can now be turned on and off with the nursery_collector_is_parallel
variable.  We will make this configurable via MONO_GC_PARAMS once the
bug is found and fixed.