a new strategy to triple the write speed of LevelDB

LevelDB 's write throughput is very poor when the DB grew large(suppose 500GB). It hurts me in my write heavy application.
In a 7 level db, to save a record should take 1 write to log, 1write to level0, about 11 writes to move up a level, that is 66 writes for it to reach level6(about 90% records is in this level).
I have tried another [level strategy](https://github.com/yedf/mergedb) to arrange data. It will only need about 15 writes. Is anyone interested?
I test it on an aliyun ECS, here is the write result:

```
mergedb: 853.131 micros/op; 4.6 MB/s
leveldb: 1913.242 micros/op; 2.0 MB/s
```