new:
1. transfer command now written to aof to sync transfer status to slave
2. each bucket now only 1 transfer client allowed to transfer the bucket

TODO:
transfer in/out command should in different format to avoid conflict when replicate to redis slave
add: modify transbegin/transend command, add in/out parameter to make the commands' function more clear.
bug fix. aof load thread process manage commands in a wrong way,fixed.
add: now buckets status can write to rdb file, but not finished.(this is a temp commit)