os/bluestore/AvlAllocator: introduce bluestore_avl_alloc_ff_max_search_bytes

os/bluestore/AvlAllocator: introduce bluestore_avl_alloc_ff_max_search_bytes

so AvlAllocator can switch from the first-first mode to best-fit mode
without walking through the whole space map tree. in the
highly-fragmented system, iterating the whole tree could hurt the
performance of fast storage system a lot.

the idea comes from openzfs's metaslab allocator.

Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit 5a26875049d13130ffe5954428da0e1b9750359f)
Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>
 Conflicts:
	src/common/options/global.yaml.in:
	- Moved new option into src/common/options.cc