RFR: Clear outgoing matrix for root regions during preparation phase

[Roman Kennke]

We're currently clearing the outgoing matrix for root regions during the
collection phase of partial GC. I find it more natural to do it in the
preparation phase. While it's not per-se incorrect (only one thread
should have to do with each root region at a time), I'm a bit uneasy to
have threads clearing whole rows, while other threads are updating the
matrix. They *should* not collide, but I'll sleep better if we could
just do it during preparation. It also means to move verification up
before preparation, because otherwise whole rows will be cleared.

http://cr.openjdk.java.net/~rkennke/cleanmatrix/webrev.00/
<http://cr.openjdk.java.net/%7Erkennke/cleanmatrix/webrev.00/>

Roman