Reviewed-by: tschatzl, pliden

Reviewed-by: tschatzl, pliden

Summary: Various cleanups as a prelude to introducing iterators for HeapRegions.
Reviewed-by: johnc, brutisso

Summary: Assertion incorrectly checks that _head is NULL and should be checking that _tail is NULL instead.
Reviewed-by: johnc
Contributed-by: NBrandon Mitchell <brandon@twitter.com>

Summary: Change the type of fields / variables / etc. that represent region counts and indeces from size_t to uint.
Reviewed-by: iveresov, brutisso, jmasa, jwilhelm

Summary: Keep track of all the old regions in the heap with a heap region set.
Reviewed-by: brutisso, johnc

Summary: Some G1 code cleanup.
Reviewed-by: johnc, jcoomes, jwilhelm

Summary: What the synopsis says.
Reviewed-by: jwilhelm, iveresov, johnc

Summary: This changeset removes the zero-filling thread from G1 and collapses the two free region lists we had before (the "free" and "unclean" lists) into one. The new free list uses the new heap region sets / lists abstractions that we'll ultimately use it to keep track of all regions in the heap. A heap region set was also introduced for the humongous regions. Finally, this change increases the concurrency between the thread that completes freeing regions (after a cleanup pause) and the rest of the system (before we'd have to wait for said thread to complete before allocating a new region). The changest also includes a lot of refactoring and code simplification.
Reviewed-by: jcoomes, johnc