6722565: G1: assert !r->is_on_unclean_list() fires
Summary: Under certain circumstances, two cleanup threads can claim and process the same region.
Reviewed-by: apetrusenko, ysr
6652160: G1: assert(cur_used_bytes == _g1->recalculate_used(),"It should!") at g1CollectorPolicy.cpp:1425
Summary: In attempt_allocation_slow() wait for the concurrent cleanup to complete before modifying _summary_bytes_used.
Reviewed-by: jmasa, apetrusenko