Add sibling explosion test

DVVset is great.

## Benchmark with CS 1.5 & Riak 1.4

.riak_test.config:
```erlang
     {sibling_benchmark,
      [{write_concurrency, 7},
       {duration_sec, 30},
       {version, previous}]
     },
```
result: max # of siblings sometimes exceeds 3x more than write concurrency
```
09:40:01.252 [info] siblings Mean:      2       0       0       0
09:40:01.252 [info] siblings Max:       21      0       0       0
09:40:01.252 [info] objsize Mean:       236805  0       0       0
09:40:01.252 [info] objsize Max:        3812184 0       0       0
09:40:01.253 [info] time Mean:  10542   0       0       0
09:40:01.253 [info] time Max:   1119368 0       0       0
09:40:01.306 [info] SiblingCount: 9, HistoryCounts: [373,373,373,373,371,372,371,371,371]
```

## Benchmark with CS develop (2.0 candidate) & Riak 2.0

.riak_test.config:
```erlang
     {sibling_benchmark,
      [{write_concurrency, 7},
       {duration_sec, 30},
       {version, current}]
     },
```
result: DVVset is great; # of siblings never exceeds write concurrency
```
09:44:30.237 [info] ====================== benchmark done ====================
09:44:30.238 [info] siblings Mean:      1       0       0       0
09:44:30.238 [info] siblings Max:       7       0       0       0
09:44:30.238 [info] objsize Mean:       109136  0       0       0
09:44:30.238 [info] objsize Max:        841313  0       0       0
09:44:30.238 [info] time Mean:  11034   0       0       0
09:44:30.238 [info] time Max:   719451  0       0       0
09:44:30.246 [info] SiblingCount: 2, HistoryCounts: [248,248]
09:44:30.246 [debug] MaxSib:Concurrency = 7:7
```

Here should be assertion. Will work on that.