move riak_kv_console:transfers to riak_core

nothing about it is specific to riak_kv
kv_vnode: expose backend size for handoff progress if backend is capable
add bitcask_backend:data_size/1 for tracking handoff progress
add memory_backend:data_size/1 for tracking handoff progress
add eleveldb_backend:data_size/1 for tracking handoff progress