reuse leveldb connection

mirage crashed and data directory was broken.

``` plain
2014/12/07 14:23:25 leveldb: could not open manifest file "MANIFEST-000606" for DB "./data": open ./data/MANIFEST-000606: no such file or directory
```

I think leveldb-go is goroutine-safe but locking database on file system does not work correctly.

https://gist.github.com/shogo82148/a643654c93cb5ae7a16a