os/bluestore: add BtreeAllocator

os/bluestore: add BtreeAllocator

because each AVL tree node needs to keep track of its children, the
overhead of each extent in AvlAllocator is relatively higher than that
of bitmap allocator, per node sizes 80 bytes, per per mempool stats provided
by unittest_alloc_bench. while btree has lower overhead, as it keeps multiple
entries in each node / block, the node size of abseil's btree implementation
defaults to 256, so each node is able to contain up to 256 values. this means
we have lower overhead than the binary-trees like AVL and red-black tree.

but because the overhead of rebalance the btree is higher than that of
binary tree, its takes more CPU cycles when performing alloc-release
than its AVL version. but its upside is that its memory layout is
more compact, under some use case, it might be a better alternative
than AvlAllocator.

Signed-off-by: Kefu Chai <kchai@redhat.com>