Chunk server: per device (disk) buffer managers: initial implementation of the required client state machine and kfs ops logic.
Chunk server: fix per disk buffer manager logic in client state machine to support io to multiple different devices, by creating device specific buffer manager client.
Chunk server: use "fast" pool allocator to allocate device buffer manager clients object, as the objects are small and there can be  large number of these.
Chunk server: fix formatting.
Merge branch 'topic/tiered_storage' into topic/disk_buffer_manager
Merge branch 'topic/tiered_storage' into topic/disk_buffer_manager
Merge branch 'topic/disk_buffer_manager' into topic/tiered_storage