Concurrent sgen improvements

[BrzVlad]

No description provided.

[schani]

This is awesome!

Sorry to make your life even more difficult, but could we please split this up into two separate pull requests, one that fixes bugs (and which we can backport) and the other one that adds features?

Bug fixes being:

[sgen] Fix scanning of invalid entries within arrays
[sgen] Clear nursery at gc if we are doing concurrent checks
[sgen] Major allocation should not happen while we sweep
[sgen] Add missing write barriers for the concurrent collector

[lewurm]

builddeb

[lewurm]

<botmode>
Benchmark results: http://xamarin.github.io/benchmarker/front-end/pullrequest.html#1UQuu6Erjl
buildbot logs: http://performancebot.mono-project.com/builders/debian-amd64_benchmarker_pullrequest_auto-sgen-noturbo/builds/20
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