G1: Lazily initialize OtherRegionsTable::_coarse_map

Eagerly initializing the per-region OtherRegionsTable::_coarse_map CHeapBitMap contributes noticeably to startup overhead in configurations with a lot of heap regions. It's relatively easy to lazily initialize the bitmap. And since most places where the _coarse_map is checked are guarded by _n_coarse_entries > 0, making this lazy should have little performance impact on GC pause times.