hybrid allocator based on btree allocator

hi Adam, while looking at Hybrid Allocator [0] and the newly introduced Btree Allocator [1], i am wondering if we still need the bitmap allocator to cap the memory usage due to the large overhead of AVL allocator? because btree is much more spatially efficient than AVL tree. cheers --- [0] https://github.com/ceph/ceph/pull/33365 [1] https://github.com/ceph/ceph/pull/41828