missing some keys temporarily during Add/Remove Server -> Rebalance

this is the son of MB-1312

• join 2 servers
• set some keys to numbers, like so...

for i in 0 1 2 3 4 5 6 7 8 9; do printf "set key_$i 0 0 1\r\n$i\r\n" | nc 127.0.0.1 11211; done

• click one server for removal and rebalance
• while it's rebalancing, do this in a loop, even manually...

for i in 0 1 2 3 4 5 6 7 8 9; do printf "incr key_$i 1\r\n" | nc 127.0.0.1 11211; done

• most of the time you should see output like...

$ for i in 0 1 2 3 4 5 6 7 8 9; do printf "incr key_$i 1\r\n" | nc 127.0.0.1 12001; done
48
49
50
51
52
53
54
55
56
57

• but, sometimes you'll see...

$ for i in 0 1 2 3 4 5 6 7 8 9; do printf "incr key_$i 1\r\n" | nc 127.0.0.1 12001; done
59
60
61
62
63
64
65
66
SERVER_ERROR a2b arith error
SERVER_ERROR a2b arith error

• and, after the rebalance is all done, the keys do come back...

$ for i in 0 1 2 3 4 5 6 7 8 9; do printf "incr key_$i 1\r\n" | nc 127.0.0.1 12001; done
61
62
63
64
65
66
67
68
63
63

• you can see values 63 were the "old data" that came back.