Allow allocation during a concurrent GC.

Allow allocation during a concurrent GC.

Previously, any thread performing a GC held the heap lock for the
entire GC.  If the GC performed was a concurrent GC, mutator threads
that allocate during the GC would be blocked until the GC completed.

With this change, if the GC performed is a concurrent GC, the heap
lock is released while the roots are being traced.  If a mutator
thread allocates an object from available storage, the allocation
proceeds.  If a mutator thread attempts to allocate an object larger
than available storage, the thread will block until the GC completes.

Change-Id: I91a04179c6f583f878b685405a6fdd16b9995017